feat(core): include pytest conftest.py in Python group

conftest.py hosts pytest fixtures and hook implementations — logically
test-adjacent rather than test-under-test, but from an ingestion budget
perspective it's still test scaffolding a user analysing production code
rarely wants indexed.

Kept as a separate opt-in suggestion so users who share module-level
fixtures across production and test paths (rare, but possible) can leave
it commented and skip only the *_test.py / test_*.py globs above.