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.

synapse/federation/transport/client.py

@@ -229,21 +229,21 @@ class TransportLayerClient:
         """
         logger.debug(
             "send_data dest=%s, txid=%s",
-            transaction.destination,  # type: ignore
-            transaction.transaction_id,  # type: ignore
+            transaction.destination,
+            transaction.transaction_id,
         )
 
-        if transaction.destination == self.server_name:  # type: ignore
+        if transaction.destination == self.server_name:
             raise RuntimeError("Transport layer cannot send to itself!")
 
         # FIXME: This is only used by the tests. The actual json sent is
         # generated by the json_data_callback.
         json_data = transaction.get_dict()
 
-        path = _create_v1_path("/send/%s", transaction.transaction_id)  # type: ignore
+        path = _create_v1_path("/send/%s", transaction.transaction_id)
 
         return await self.client.put_json(
-            transaction.destination,  # type: ignore
+            transaction.destination,
             path=path,
             data=json_data,
             json_data_callback=json_data_callback,