fix: correct python_versions() minimum for major-only and multiple lower bounds

[Sanjays2402]

Summary

nox.project.python_versions(pyproject, max_version=...) reads the lower
bound of supported Python versions from requires-python. It looped over the
SpecifierSet and broke on the first lower-bound specifier it encountered,
reading the minor version with spec.version.split(".")[1]. That logic has two
bugs:

1. Crash on a major-only version

A valid requires-python = ">=3" (equivalent to ">=3.0" per PEP 440) has no
minor component, so "3".split(".")[1] raises a cryptic IndexError straight
out of this public helper:

>>> import nox.project
>>> nox.project.python_versions({"project": {"requires-python": ">=3"}}, max_version="3.13")
Traceback (most recent call last):
  ...
IndexError: list index out of range

2. Wrong result with multiple lower bounds

For requires-python = ">=3.8,>=3.10" the effective minimum is 3.10, but
breaking on the first lower-bound specifier returns 3.8, so the result claims
support for 3.8 and 3.9 which the metadata explicitly excludes:

>>> nox.project.python_versions({"project": {"requires-python": ">=3.8,>=3.10"}}, max_version="3.12")
['3.8', '3.9', '3.10', '3.11', '3.12']   # should be ['3.10', '3.11', '3.12']

The output could even flip depending on specifier iteration order.

Root cause

Both symptoms come from the same block, which assumed (a) exactly one relevant
lower bound and (b) that a version string always has a major.minor shape.

Fix

Collect every lower-bound (>, >=, ~=) specifier's minor version,
treat a major-only version as minor 0, and use the maximum as the
effective minimum. When no lower-bound specifier exists, the existing
ValueError is still raised.

requires-python	before	after
">=3"	IndexError	['3.0', …, max]
">=3.8,>=3.10" (max 3.12)	['3.8','3.9','3.10','3.11','3.12']	['3.10','3.11','3.12']
">=3.10,>=3.8" (max 3.12)	order-dependent	['3.10','3.11','3.12']
">=3.10" (max 3.12)	['3.10','3.11','3.12']	unchanged
">3.2.1,<3.3" (max 3.4)	['3.2','3.3','3.4']	unchanged
"==3.3.1"	ValueError	unchanged

Tests

Added test_python_range_major_only and test_python_range_multiple_lower_bounds
to tests/test_project.py. Verified they fail on main (the first with the
IndexError, the second asserting on the wrong ['3.8', '3.9', …] output) and
pass with the fix.

$ python -m pytest tests/test_project.py -q
9 passed

$ python -m pytest -q      # full suite
720 passed, 27 skipped, 1 xpassed

ruff check and ruff format --check are clean on the changed files, and the
change introduces no new mypy errors (the 3 reported in nox/project.py are
pre-existing and outside this diff).