ci: experimental MSYS2/MinGW test run

.github/ISSUE_TEMPLATE/bug.yml

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+name: 🐞 Bug Report
+description: File a bug/issue
+title: "<title>"
+labels:
+  - "bug"
+body:
+  - type: textarea
+    attributes:
+      label: Current Behavior
+      description: A concise description of what you're experiencing.
+    validations:
+      required: false
+  - type: textarea
+    attributes:
+      label: Expected Behavior
+      description: A concise description of what you expected to happen.
+    validations:
+      required: false
+  - type: textarea
+    attributes:
+      label: Steps To Reproduce
+      description: Steps to reproduce the behavior.
+      placeholder: |
+        1. With this Nox session/file...
+        2. Run '...'
+        3. See error...
+    validations:
+      required: false
+  - type: textarea
+    attributes:
+      label: Environment
+      description: |
+        examples:
+          - **OS**: Ubuntu 24.04
+          - **Python**: 3.14.3
+          - **Nox**: 2026.04.10
+      value: |
+        - OS:
+        - Python:
+        - Nox:
+      render: Markdown
+    validations:
+      required: false
+  - type: textarea
+    attributes:
+      label: Anything else?
+      description: |
+        Links? References? Anything that will give us more context about the issue you are encountering!
+
+        Tip: You can attach images or log files by clicking this area to highlight it and then dragging files in.
+    validations:
+      required: false

.github/ISSUE_TEMPLATE/config.yml

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+contact_links:
+  - name: Winterbloom Discord server
+    url: https://discord.gg/C5EH3N3fsk
+    about: You can ask questions and chat about Nox under this server's '#nox' channel.

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: 🚀 Feature Request
+description: Request a new feature or enhancement
+title: "<title>"
+labels:
+  - "enhancement"
+body:
+  - type: textarea
+    attributes:
+      label: How would this feature be useful?
+      description: Describe any use cases this solves or frustrations it alleviates.
+    validations:
+      required: false
+
+  - type: textarea
+    attributes:
+      label: Describe the solution you'd like
+      description: If you have an idea on how to do this, let us know here!
+    validations:
+      required: false
+
+  - type: textarea
+    attributes:
+      label: Describe alternatives you've considered
+      description: If there's some workaround or alternative solutions, let us know here!
+    validations:
+      required: false
+
+  - type: textarea
+    attributes:
+      label: Anything else?
+      description: Any other relevant information or background.
+    validations:
+      required: false

.github/action_helper.py

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+from __future__ import annotations
+
+import json
+import platform
+import sys
+
+
+def filter_version(version: str) -> str:
+    """return python 'major.minor'"""
+
+    # remove interpreter prefix
+    if version.startswith("pypy-"):
+        version_ = version[5:]
+    elif version.startswith("pypy"):
+        version_ = version[4:]
+    elif version.startswith("~"):
+        version_ = version[1:]
+    else:
+        version_ = version
+
+    # remove extra specifier e.g. "3.12-dev" => "3.12", "~3.12.0-0" => "3.12"
+    version_ = version_.split("-")[0]
+
+    # Handle free-threaded
+    free_threaded = version_.endswith("t")
+    if free_threaded:
+        version_ = version_[:-1]
+
+    version_parts = version_.split(".")
+    if len(version_parts) < 2:
+        msg = f"invalid version: {version}"
+        raise ValueError(msg)
+    if not version_parts[0].isdigit():
+        msg = f"invalid major python version: {version}"
+        raise ValueError(msg)
+    if not version_parts[1].isdigit():
+        msg = f"invalid minor python version: {version}"
+        raise ValueError(msg)
+    return ".".join(version_parts[:2]) + ("t" if free_threaded else "")
+
+
+def valid_version(version: str) -> bool:
+    if sys.platform.startswith("win32") and platform.machine().lower() in {
+        "arm64",
+        "aarch64",
+    }:
+        if version in {"2.7", "3.6", "3.7", "3.8", "3.9", "3.10"}:
+            return False
+    return True
+
+
+def setup_action(input_: str, *, self_version: str = "3.12") -> None:
+    versions = [version.strip() for version in input_.split(",") if version.strip()]
+
+    pypy_versions = [version for version in versions if version.startswith("pypy")]
+    pypy_versions_filtered = [filter_version(version) for version in pypy_versions]
+    if len(pypy_versions) != len(set(pypy_versions_filtered)):
+        msg = (
+            "multiple versions specified for the same 'major.minor' PyPy interpreter:"
+            f" {pypy_versions}"
+        )
+        raise ValueError(msg)
+
+    cpython_versions = [version for version in versions if version not in pypy_versions]
+    cpython_versions_filtered = [
+        filter_version(version) for version in cpython_versions
+    ]
+    if len(cpython_versions) != len(set(cpython_versions_filtered)):
+        msg = (
+            "multiple versions specified for the same 'major.minor' CPython"
+            f" interpreter: {cpython_versions}"
+        )
+        raise ValueError(msg)
+
+    # cpython shall be installed last because
+    # other interpreters also define pythonX.Y symlinks.
+    versions = [v for v in pypy_versions + cpython_versions if valid_version(v)]
+
+    # we want to install our own self version last to ease nox set-up
+    if self_version in cpython_versions_filtered:
+        index = cpython_versions_filtered.index(self_version)
+        index = versions.index(cpython_versions[index])
+        cpython_nox = versions.pop(index)
+        versions.append(cpython_nox)
+    else:
+        # add this to install nox
+        versions.append(self_version)
+
+    print(f"interpreters={json.dumps(versions)}")
+
+
+if __name__ == "__main__":
+    if len(sys.argv) != 2:
+        msg = f"invalid arguments: {sys.argv}"
+        raise AssertionError(msg)
+    setup_action(sys.argv[1])

.github/dependabot.yml

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+version: 2
+
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+    cooldown:
+      default-days: 7
+
+  - package-ecosystem: "pre-commit"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      pre-commit:
+        patterns:
+          - "*"
+    cooldown:
+      default-days: 7

.github/release.yml

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+changelog:
+  exclude:
+    authors:
+      - dependabot[bot]

.github/workflows/action.yml

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+name: Action
+on:
+  push:
+    branches:
+      - "main"
+      - "**action**"
+  pull_request:
+    paths:
+      - ".github/workflows/action.yml"
+      - ".github/action_helper.py"
+      - "action.yml"
+  workflow_dispatch:
+    # allow manual runs on branches without a PR
+
+env:
+  FORCE_COLOR: "1"
+
+permissions: {}
+
+jobs:
+  action-default-tests:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os:
+          - ubuntu-24.04
+          - ubuntu-24.04-arm
+          - windows-2022
+          - windows-2025
+          - windows-11-arm
+          - macos-15-intel
+          - macos-15
+    steps:
+      - uses: actions/checkout@v6.0.3
+        with:
+          persist-credentials: false
+      - uses: astral-sh/setup-uv@v8.2.0
+      - uses: ./
+      - run: nox --session github_actions_default_tests
+
+  action-all-tests:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v6.0.3
+        with:
+          persist-credentials: false
+      - uses: actions/setup-python@v6
+        with:
+          python-version: 3.12
+      - uses: ./
+        with:
+          python-versions: "pypy-2.7, pypy-3.8, pypy-3.9, pypy-3.10, pypy-3.11, 3.6, 3.7, 3.8, 3.9, 3.10, 3.11, 3.12, 3.13, 3.14, 3.13t, 3.14t"
+      - run: nox --session github_actions_all_tests

.github/workflows/cd.yml

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+name: CD
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+  release:
+    types: [published]
+
+env:
+  FORCE_COLOR: "1"
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions: {}
+
+jobs:
+  dist:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6.0.3
+        with:
+          persist-credentials: false
+      - uses: hynek/build-and-inspect-python-package@v2
+
+  deploy:
+    needs: [dist]
+    environment:
+      name: pypi
+      url: https://pypi.org/project/nox
+    permissions:
+      id-token: write
+      attestations: write
+    runs-on: ubuntu-latest
+    if: github.event_name == 'release' && github.event.action == 'published'
+    steps:
+      - uses: actions/download-artifact@v8
+        with:
+          name: Packages
+          path: dist
+
+      - name: Generate artifact attestation for sdist and wheel
+        uses: actions/attest@v4
+        with:
+          subject-path: "dist/*"
+
+      - uses: pypa/gh-action-pypi-publish@release/v1