[3.13] gh-129668: Fix thread-safety of MemoryError freelist in free threaded build (gh-129704) (gh-129742)

The MemoryError freelist was not thread-safe in the free threaded build.
Use a mutex to protect accesses to the freelist. Unlike other freelists,
the MemoryError freelist is not performance sensitive.

(cherry picked from commit 51b4edb1a4092f60d84f7d14eb41c12085e39c31)