Fix data race detected by tsan (https://github.com/python/cpython/actions/runs/14857021107/job/41712717208?pr=133502): young.count can be modified by other threads even while the gcstate is locked. This is the simplest fix to (potentially) unblock beta 1, although this particular code path seems like it could just be an atomic swap followed by an atomic add, without having the lock at all.
Miscellaneous source files for the main Python shared library