bpo-26510: Add versionchanged for required arg of add_subparsers (GH-16588)

The `required` argument to `argparse.add_subparsers` was added in GH-3027. This PR specifies the earliest version of Python where it is available.

https://bugs.python.org/issue26510

Automerge-Triggered-By: @merwok
(cherry picked from commit 9e71917e02)

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

Doc/library/argparse.rst

@@ -1583,7 +1583,7 @@ Sub-commands
      stored; by default ``None`` and no value is stored
 
    * required_ - Whether or not a subcommand must be provided, by default
-     ``False``.
+     ``False`` (added in 3.7)
 
    * help_ - help for sub-parser group in help output, by default ``None``
 
@@ -1739,6 +1739,9 @@ Sub-commands
      >>> parser.parse_args(['2', 'frobble'])
      Namespace(subparser_name='2', y='frobble')
 
+   .. versionchanged:: 3.7
+      New *required* keyword argument.
+
 
 FileType objects
 ^^^^^^^^^^^^^^^^

Doc/whatsnew/3.7.rst

@@ -2414,6 +2414,10 @@ Changes in the Python API
   instead of a :class:`bytes` instance.
   (Contributed by Victor Stinner in :issue:`21071`.)
 
+* :mod:`argparse` subparsers can now be made mandatory by passing ``required=True``
+  to :meth:`ArgumentParser.add_subparsers() <argparse.ArgumentParser.add_subparsers>`.
+  (Contributed by Anthony Sottile in :issue:`26510`.)
+
 * :meth:`ast.literal_eval()` is now stricter.  Addition and subtraction of
   arbitrary numbers are no longer allowed.
   (Contributed by Serhiy Storchaka in :issue:`31778`.)