GH-110109: Drop use of new regex features in pathlib._abc.

[barneygale]

A regex group with a ?+ possessive quantifier was used to match empty paths, which were represented as a single dot, preventing the dot from being matched by other wildcards. This quantifier is only available from Python 3.11+, but pathlib's _abc.py file will be made available as a PyPI package for Python 3.8+.

This commit adds a new private _pattern_str property that works like __str__() but represents empty paths as '' rather than '.'. This string is used for pattern matching, which removes the need for the possessive group. We also replace re.NOFLAG with 0, again for backwards-compatibility with older Python.

Improves compatibility with older Python; no other change of behaviour.

See external pathlib_abc issue: barneygale/pathlib-abc#8

• Issue: User-defined virtual path objects #110109

[barneygale]

I think this is slower, but only very slightly:

$ ./python -m timeit -s "from pathlib import Path; p = Path('foo.py')" "p.match('*.py')"
50000 loops, best of 5: 4.75 usec per loop  # before
50000 loops, best of 5: 4.81 usec per loop  # after

$ ./python -m timeit -s "from pathlib import Path" "list(Path.cwd().glob('**/*', follow_symlinks=True))"
5 loops, best of 5: 81 msec per loop  # before
5 loops, best of 5: 81 msec per loop  # after

[barneygale]

@AlexWaygood pointed me towards this video, which shows how to implement atomic groups/posessive qualifiers in older Python using a capturing lookahead and a backref. It's a one-line patch so I'll apply it only in the external pathlib_abc package. Withdrawing this PR.