requirements: Drop direct dependency on mock.

mock is just a backport of the standard library’s unittest.mock now.

The SAMLAuthBackendTest change is needed because
MagicMock.call_args.args wasn’t introduced until Python
3.8 (https://bugs.python.org/issue21269).

The PROVISION_VERSION bump is skipped because mock is still an
indirect dev requirement via moto.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/test_classes.py

@@ -68,7 +68,7 @@ from zerver.decorator import do_two_factor_login
 from zerver.tornado.event_queue import clear_client_event_queues_for_testing
 
 import base64
-import mock
+from unittest import mock
 import os
 import re
 import ujson