gh-116510: Fix crash during sub-interpreter shutdown (gh-124645)
Fix a bug that can cause a crash when sub-interpreters use "basic"
single-phase extension modules. Shared objects could refer to PyGC_Head
nodes that had been freed as part of interpreter shutdown.
(cherry picked from commit 6f9525dd3f)
Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>