[3.14] gh-146270: Fix `PyMember_SetOne(..., NULL)` not being atomic (gh-148800) (#149460)

Fixes a sequential consistency bug whereby two threads that are deleting a struct member may observe both their deletions to be successful.

(cherry picked from commit 1bdfc0f253730077ccd3a4b0714388e8227b1b71)

Co-authored-by: Daniele Parmeggiani <8658291+dpdani@users.noreply.github.com>