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 <zspytz@gmail.com>

Author: jaraco

Doc/library/importlib.metadata.rst

@@ -7,6 +7,8 @@
 .. module:: importlib.metadata
  :synopsis: The implementation of the importlib metadata.
 
+.. versionadded:: 3.8
+
 **Source code:** :source:`Lib/importlib/metadata.py`
 
 .. note::