Implement optional importlib.abc.MetaPathFinder.invalidate_caches()

[moreati]

mitogen.core.Importer originally implements importlib.abc.Finder, with one method find_module(fullname, path=None). Finder was deperecated in Python 3.3, removed in 3.12, replaced by importlib.abc.MetaPathFinder and importlib.abc.PathEntryFinder. Both have an optional invalidate_caches() method.