Doc: Remove old Python version from future stmt (GH-21802)

(cherry picked from commit eba109a)

Co-authored-by: Andre Delfino <[email protected]>

Author: miss-islington

Doc/reference/simple_stmts.rst

@@ -874,8 +874,8 @@ can appear before a future statement are:
 * blank lines, and
 * other future statements.
 
-The only feature in Python 3.7 that requires using the future statement is
-``annotations``.
+The only feature that requires using the future statement is
+``annotations`` (see :pep:`563`).
 
 All historical features enabled by the future statement are still recognized
 by Python 3.  The list includes ``absolute_import``, ``division``,