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>

tools/test-backend

@@ -8,7 +8,7 @@ from typing import List, Iterator
 import glob
 import argparse
 import contextlib
-import mock
+from unittest import mock
 import os
 import shlex
 import sys