Use mock from the stdlib. (#9772)

tests/events/test_presence_router.py

@@ -13,8 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 from typing import Dict, Iterable, List, Optional, Set, Tuple, Union
-
-from mock import Mock
+from unittest.mock import Mock
 
 import attr
 
@@ -314,7 +313,8 @@ class PresenceRouterTestCase(FederatingHomeserverTestCase):
             self.hs.get_federation_transport_client().send_transaction.call_args_list
         )
         for call in calls:
-            federation_transaction = call.args[0]  # type: Transaction
+            call_args = call[0]
+            federation_transaction = call_args[0]  # type: Transaction
 
             # Get the sent EDUs in this transaction
             edus = federation_transaction.get_dict()["edus"]