gh-116811: Ensure MetadataPathFinder.invalidate_caches is reachable when delegated through PathFinder. (#116812)

* Make MetadataPathFinder a proper classmethod.

* In PathFinder.invalidate_caches, also invoke MetadataPathFinder.invalidate_caches.

* Add blurb

Lib/importlib/_bootstrap_external.py

@@ -1470,6 +1470,9 @@ class PathFinder:
         # https://bugs.python.org/issue45703
         _NamespacePath._epoch += 1
 
+        from importlib.metadata import MetadataPathFinder
+        MetadataPathFinder.invalidate_caches()
+
     @staticmethod
     def _path_hooks(path):
         """Search sys.path_hooks for a finder for 'path'."""

Lib/importlib/metadata/__init__.py

@@ -797,6 +797,7 @@ class MetadataPathFinder(DistributionFinder):
             path.search(prepared) for path in map(FastPath, paths)
         )
 
+    @classmethod
     def invalidate_caches(cls) -> None:
         FastPath.__new__.cache_clear()
 

Misc/NEWS.d/next/Library/2024-03-14-10-01-23.gh-issue-116811._h5iKP.rst

@@ -0,0 +1,2 @@
+In ``PathFinder.invalidate_caches``, delegate to
+``MetadataPathFinder.invalidate_caches``.