[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 5f52d20a93908196f74271db8437cc1ba7e1e262)