Fix up type hints for Twisted 21.7 (#10490)

Mostly this involves decorating a few Deferred declarations with extra type hints. We wrap the types in quotes to avoid runtime errors when running against older versions of Twisted that don't have generics on Deferred.

synapse/util/async_helpers.py

@@ -49,6 +49,8 @@ from synapse.util import Clock, unwrapFirstError
 
 logger = logging.getLogger(__name__)
 
+_T = TypeVar("_T")
+
 
 class ObservableDeferred:
     """Wraps a deferred object so that we can add observer deferreds. These
@@ -121,7 +123,7 @@ class ObservableDeferred:
         effect the underlying deferred.
         """
         if not self._result:
-            d = defer.Deferred()
+            d: "defer.Deferred[Any]" = defer.Deferred()
 
             def remove(r):
                 self._observers.discard(d)
@@ -415,7 +417,7 @@ class ReadWriteLock:
         self.key_to_current_writer: Dict[str, defer.Deferred] = {}
 
     async def read(self, key: str) -> ContextManager:
-        new_defer = defer.Deferred()
+        new_defer: "defer.Deferred[None]" = defer.Deferred()
 
         curr_readers = self.key_to_current_readers.setdefault(key, set())
         curr_writer = self.key_to_current_writer.get(key, None)
@@ -438,7 +440,7 @@ class ReadWriteLock:
         return _ctx_manager()
 
     async def write(self, key: str) -> ContextManager:
-        new_defer = defer.Deferred()
+        new_defer: "defer.Deferred[None]" = defer.Deferred()
 
         curr_readers = self.key_to_current_readers.get(key, set())
         curr_writer = self.key_to_current_writer.get(key, None)
@@ -471,10 +473,8 @@ R = TypeVar("R")
 
 
 def timeout_deferred(
-    deferred: defer.Deferred,
-    timeout: float,
-    reactor: IReactorTime,
-) -> defer.Deferred:
+    deferred: "defer.Deferred[_T]", timeout: float, reactor: IReactorTime
+) -> "defer.Deferred[_T]":
     """The in built twisted `Deferred.addTimeout` fails to time out deferreds
     that have a canceller that throws exceptions. This method creates a new
     deferred that wraps and times out the given deferred, correctly handling
@@ -497,7 +497,7 @@ def timeout_deferred(
     Returns:
         A new Deferred, which will errback with defer.TimeoutError on timeout.
     """
-    new_d = defer.Deferred()
+    new_d: "defer.Deferred[_T]" = defer.Deferred()
 
     timed_out = [False]