gh-125245: Fix race condition when importing `collections.abc` (#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.

Lib/collections/__init__.py

@@ -29,6 +29,9 @@ __all__ = [
 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

@@ -1,3 +0,0 @@
-import _collections_abc
-import sys
-sys.modules[__name__] = _collections_abc

Misc/NEWS.d/next/Library/2024-10-11-00-40-13.gh-issue-125245.8vReM-.rst

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