Fix up some typechecking (#6150)

* type checking fixes

* changelog

synapse/api/room_versions.py

@@ -12,6 +12,9 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+from typing import Dict
+
 import attr
 
 
@@ -102,4 +105,4 @@ KNOWN_ROOM_VERSIONS = {
         RoomVersions.V4,
         RoomVersions.V5,
     )
-}  # type: dict[str, RoomVersion]
+}  # type: Dict[str, RoomVersion]