Fix mock import in tests.

For some reason, one test imports Mock class from mock.mock
rather than from mock.
This change fixes this error.

Signed-off-by: Oleg Girko <ol@infoserver.lv>

tests/storage/test_events.py

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import uuid
-from mock.mock import Mock
+from mock import Mock
 from synapse.types import RoomID, UserID
 
 from tests import unittest