[3.12] gh-116811: Ensure MetadataPathFinder.invalidate_caches is reachable when delegated through PathFinder. (GH-116812) (#116864)

* Make MetadataPathFinder a proper classmethod.

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

* Add blurb
(cherry picked from commit 5f52d20a93)

Lib/importlib/_bootstrap_external.py

@@ -1450,6 +1450,9 @@ 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'."""