[3.9] bpo-43856: Add a versionadded directive to the importlib.metadata docs (GH-25445)

Use a versionadded directive to generate the text "New in version
3.8." (to match with the documentation of other modules).

Automerge-Triggered-By: GH:jaraco.
(cherry picked from commit adf24bd)

Co-authored-by: Zackery Spytz <[email protected]>

Author: ZackerySpytz

Doc/library/importlib.metadata.rst

@@ -4,6 +4,8 @@
  Using :mod:`!importlib.metadata`
 =================================
 
+.. versionadded:: 3.8
+
 .. note::
    This functionality is provisional and may deviate from the usual
    version semantics of the standard library.