fix: handle Python 3.15 removal of the legacy sre_parse module

[Qubitium]

Problem

Python 3.15 removes the deprecated top-level sre_parse / sre_compile / sre_constants modules (previously slated for removal, now gone in 3.15.0rc1). pypcre's parser loader in pcre/_stdlib_re.py imported sre_parse unconditionally whenever re._parser was absent, so on 3.15 that path failed with an opaque ModuleNotFoundError: No module named 'sre_parse'.

Found by running the full test suite against Python 3.15.0rc1: tests/test_python_coverage_audit.py::test_stdlib_parser_fallback failed; everything else passed.

Fix

• pcre/_stdlib_re.py: guard the legacy fallback import with try/except ModuleNotFoundError and raise a clear ImportError naming both supported internals (re._parser on Python 3.11+, sre_parse on Python <= 3.10) when neither exists.
• tests/test_python_coverage_audit.py::test_stdlib_parser_fallback: version-aware expectations — return/validate a parser where sre_parse still exists (<= 3.14), expect ImportError where it has been removed (3.15+).

Forward-compatibility verification on 3.15.0rc1

• Full suite: 832 passed / 19 skipped; only failure is test_cache_strategy_benchmark, which fails on pristine main as well (timing-sensitive).
• New-in-3.15 re.prefixmatch() / re.Pattern.prefixmatch(): signatures fingerprint-match pypcre's existing aliases and span behavior matches re (including positional pos).
• All module-level signature parity tests pass against the 3.15 stdlib.
• import pcre and hot paths run clean under -W error::DeprecationWarning on 3.15.