Fix a type annotation in test_account_data.py and remove it from the Mypy exclusion list.

changelog.d/11657.misc

@@ -0,0 +1 @@
+Fix a type annotation in `test_account_data.py` and remove it from the Mypy exclusion list.

mypy.ini

@@ -100,7 +100,6 @@ exclude = (?x)
    |tests/server.py
    |tests/server_notices/test_resource_limits_server_notices.py
    |tests/state/test_v2.py
-   |tests/storage/test_account_data.py
    |tests/storage/test_background_update.py
    |tests/storage/test_base.py
    |tests/storage/test_client_ips.py

tests/storage/test_account_data.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Iterable, Set
+from typing import Iterable, Optional, Set
 
 from synapse.api.constants import AccountDataTypes
 
@@ -25,7 +25,7 @@ def prepare(self, hs, reactor, clock):
         self.user = "@user:test"
 
     def _update_ignore_list(
-        self, *ignored_user_ids: Iterable[str], ignorer_user_id: str = None
+        self, *ignored_user_ids: Iterable[str], ignorer_user_id: Optional[str] = None
     ) -> None:
         """Update the account data to block the given users."""
         if ignorer_user_id is None: