Remove unused # type: ignores (#12531)

Over time we've begun to use newer versions of mypy, typeshed, stub
packages---and of course we've improved our own annotations. This makes
some type ignore comments no longer necessary. I have removed them.

There was one exception: a module that imports `select.epoll`. The
ignore is redundant on Linux, but I've kept it ignored for those of us
who work on the source tree using not-Linux. (#11771)

I'm more interested in the config line which enforces this. I want
unused ignores to be reported, because I think it's useful feedback when
annotating to know when you've fixed a problem you had to previously
ignore.

* Installing extras before typechecking

Lacking an easy way to install all extras generically, let's bite the bullet and
make install the hand-maintained `all` extra before typechecking.

Now that https://github.com/matrix-org/backend-meta/pull/6 is merged to
the release/v1 branch.

tests/replication/test_federation_sender_shard.py

@@ -102,8 +102,8 @@ class FederationSenderTestCase(BaseMultiWorkerStreamTestCase):
         for i in range(20):
             server_name = "other_server_%d" % (i,)
             room = self.create_room_with_remote_server(user, token, server_name)
-            mock_client1.reset_mock()  # type: ignore[attr-defined]
-            mock_client2.reset_mock()  # type: ignore[attr-defined]
+            mock_client1.reset_mock()
+            mock_client2.reset_mock()
 
             self.create_and_send_event(room, UserID.from_string(user))
             self.replicate()
@@ -167,8 +167,8 @@ class FederationSenderTestCase(BaseMultiWorkerStreamTestCase):
         for i in range(20):
             server_name = "other_server_%d" % (i,)
             room = self.create_room_with_remote_server(user, token, server_name)
-            mock_client1.reset_mock()  # type: ignore[attr-defined]
-            mock_client2.reset_mock()  # type: ignore[attr-defined]
+            mock_client1.reset_mock()
+            mock_client2.reset_mock()
 
             self.get_success(
                 typing_handler.started_typing(