gh-125245: Fix race condition when importing collections.abc (GH-125415)

If multiple threads concurrently imported `collections.abc`, some of the
threads might incorrectly see the "shim" `Lib/collections/abc.py` module
instead of the correct `Lib/_collections_abc.py` module.  This affected
both the free threading build and the default GIL-enabled build.
(cherry picked from commit fed501d)

Co-authored-by: Sam Gross <[email protected]>

Author: colesbury

Lib/collections/__init__.py

@@ -29,6 +29,9 @@
 import _collections_abc
 import sys as _sys
 
+_sys.modules['collections.abc'] = _collections_abc
+abc = _collections_abc
+
 from itertools import chain as _chain
 from itertools import repeat as _repeat
 from itertools import starmap as _starmap

Lib/collections/abc.py

@@ -0,0 +1,2 @@
+Fix race condition when importing :mod:`collections.abc`, which could
+incorrectly return an empty module.