importlib.resouces: add as_file for Python 3.9 (#5216)

Author: henryiii

stdlib/importlib/resources.pyi

@@ -19,5 +19,7 @@ if sys.version_info >= (3, 7):
     def contents(package: Package) -> Iterator[str]: ...
 
 if sys.version_info >= (3, 9):
+    from contextlib import AbstractContextManager
     from importlib.abc import Traversable
     def files(package: Package) -> Traversable: ...
+    def as_file(path: Traversable) -> AbstractContextManager[Path]: ...