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/tests/test_presence.py

@@ -1,6 +1,6 @@
 from datetime import timedelta
 from django.utils.timezone import now as timezone_now
-import mock
+from unittest import mock
 
 from typing import Any, Dict
 from zerver.lib.actions import do_deactivate_user