Fix "versionchanged" for pow named arguments (GH-19042) (GH-19079)

The ability to use named arguments in "pow" was introduced in Python 3.8, not Python 3.9. See https://bugs.python.org/issue38237
(cherry picked from commit c691f20)

Co-authored-by: Mark Dickinson <[email protected]>

Author: miss-islington

Doc/library/functions.rst

@@ -1304,7 +1304,7 @@ are always available.  They are listed here in alphabetical order.
       the second argument to be negative, permitting computation of modular
       inverses.
 
-   .. versionchanged:: 3.9
+   .. versionchanged:: 3.8
       Allow keyword arguments.  Formerly, only positional arguments were
       supported.