Modernize build/install to pyproject.toml

[facundobatista]

The setup.py was over a decade old and carried a lot of legacy cruft:
distutils.core.setup (removed from the stdlib in 3.12), a hand-rolled regex
version parser, and a CustomInstall class doing platform-specific script
selection plus manual man-page copying. This migrates everything to a
declarative pyproject.toml (PEP 621, setuptools backend).

What changed

Packaging

• New pyproject.toml replaces setup.py for all metadata.
• Version is now read dynamically from fades/_version.py:__version__
  (no more regex parsing).
• The fades command is now a console-scripts entry point
  (fades.main:console_entry_point) instead of the shipped bin/fades /
  bin/fades.cmd scripts. setuptools generates the right launcher per platform
  (including fades.exe on Windows), so the whole CustomInstall /
  platform-selection logic is gone.
• Added console_entry_point() in fades/main.py (handles FadesError);
  python -m fades now reuses it, so there is a single entry path.

Dependency fixes

• Dropped setuptools from the runtime dependencies — fades never imports it,
  and it's irrelevant to the child venvs it builds.
• Promoted packaging from an optional extra to a hard dependency: it is
  imported unconditionally at module top level (parsing.py, helpers.py), so
  pip install fades previously produced a broken install unless packaging
  happened to already be present. pyxdg stays an extra (imported lazily, with
  a fallback).

The one remaining setup.py

• A 3-line shim survives, solely to install the man page under
  share/man/man1 via data_files. PEP 621 has no equivalent, and modern
  setuptools rejects data_files in pyproject.toml. It's consumed by
  pip --prefix, the Debian pybuild flow and the snap build.

Callers updated

• test, testdev, testdev.bat, build_readme → python -m fades
  (build_readme no longer relies on setup.py --long-description).
• .github/workflows/integtests.yaml → runs via -m fades; the minimum
  supported Python is now read from requires-python in pyproject.toml.
• HOWTO_RELEASE.txt → python -m build instead of ./setup.py clean sdist.

Verification

• python -m build produces a valid sdist + wheel; sdist includes man/,
  COPYING, AUTHORS, README.rst.
• Fresh pip install of the wheel pulls packaging automatically and does
  not install setuptools.
• fades -V and python -m fades -V both work; a usage error exits non-zero
  (FadesError handled).
• Man page installed to <prefix>/share/man/man1/fades.1.
• flake8 + pydocstyle clean; full unit suite passes (291 tests).