fix(python): tolerate older virtualenv PythonSpec missing machine/free_threaded

[SAY-5]

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

Fixes #3934.

_validate_base_python iterates over ("implementation", "major", "minor", "micro", "architecture", "machine", "free_threaded") and unconditionally calls getattr(spec_name, key) on the PythonSpec returned by virtualenv. The machine and free_threaded attributes were introduced in newer virtualenv releases, so on systems that still ship an older virtualenv (e.g. OpenIndiana's system package, virtualenv 20.37.0), tox -e py314 aborts with:

AttributeError: 'PythonSpec' object has no attribute 'machine'

Switching the lookups to getattr(spec, key, None) makes the comparison safely treat the missing keys as "no constraint", restoring the pre-3815 behaviour for environments that have not upgraded virtualenv yet. Added a regression test that monkey-patches PythonSpec.from_string_spec to return an attribute-stripped spec.

[gaborbernat]

Wouldn't this imply that on those environments we would not satisfy potentially the requirement? I would rather fail loud, like we do now, than silently do the wrong thing. People should upgrade their virtual lamp, and we should specifically require that, rather than this kind of fall back to bad behavior.