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_bots.py

@@ -4,7 +4,7 @@ import ujson
 
 from django.core import mail
 from django.test import override_settings
-from mock import patch, MagicMock
+from unittest.mock import patch, MagicMock
 from typing import Any, Dict, List, Mapping, Optional
 
 from zerver.lib.actions import do_change_stream_invite_only, do_deactivate_user, \