Fix up some typechecking (#6150)

* type checking fixes

* changelog

synapse/util/async_helpers.py

@@ -13,9 +13,11 @@
 # 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.
+
 import collections
 import logging
 from contextlib import contextmanager
+from typing import Dict, Sequence, Set, Union
 
 from six.moves import range
 
@@ -213,7 +215,9 @@ class Linearizer(object):
         # the first element is the number of things executing, and
         # the second element is an OrderedDict, where the keys are deferreds for the
         # things blocked from executing.
-        self.key_to_defer = {}
+        self.key_to_defer = (
+            {}
+        )  # type: Dict[str, Sequence[Union[int, Dict[defer.Deferred, int]]]]
 
     def queue(self, key):
         # we avoid doing defer.inlineCallbacks here, so that cancellation works correctly.
@@ -340,10 +344,10 @@ class ReadWriteLock(object):
 
     def __init__(self):
         # Latest readers queued
-        self.key_to_current_readers = {}
+        self.key_to_current_readers = {}  # type: Dict[str, Set[defer.Deferred]]
 
         # Latest writer queued
-        self.key_to_current_writer = {}
+        self.key_to_current_writer = {}  # type: Dict[str, defer.Deferred]
 
     @defer.inlineCallbacks
     def read(self, key):