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

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.