mypy: Fix type errors relate to 'mock'.

1. The type of a mock object should be 'Any'.
2. 'mock' is being imported in the wrong manner.

zerver/tests/test_decorators.py

@@ -487,7 +487,7 @@ body:
                                status_code=401)
 
 class RateLimitTestCase(TestCase):
-    def errors_disallowed(self) -> mock:
+    def errors_disallowed(self) -> Any:
         # Due to what is probably a hack in rate_limit(),
         # some tests will give a false positive (or succeed
         # for the wrong reason), unless we complain

zerver/tests/test_presence.py

@@ -4,7 +4,7 @@ from datetime import timedelta
 from django.http import HttpResponse
 from django.test import override_settings
 from django.utils.timezone import now as timezone_now
-from mock import mock
+import mock
 
 from typing import Any, Dict
 from zerver.lib.actions import do_deactivate_user