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/storage/databases/main/monthly_active_users.py

@@ -232,10 +232,10 @@ class MonthlyActiveUsersWorkerStore(RegistrationWorkerStore):
             # is racy.
             # Have resolved to invalidate the whole cache for now and do
             # something about it if and when the perf becomes significant
-            self._invalidate_all_cache_and_stream(  # type: ignore[attr-defined]
+            self._invalidate_all_cache_and_stream(
                 txn, self.user_last_seen_monthly_active
             )
-            self._invalidate_cache_and_stream(txn, self.get_monthly_active_count, ())  # type: ignore[attr-defined]
+            self._invalidate_cache_and_stream(txn, self.get_monthly_active_count, ())
 
         reserved_users = await self.get_registered_reserved_users()
         await self.db_pool.runInteraction(
@@ -363,7 +363,7 @@ class MonthlyActiveUsersWorkerStore(RegistrationWorkerStore):
 
         if self._limit_usage_by_mau or self._mau_stats_only:
             # Trial users and guests should not be included as part of MAU group
-            is_guest = await self.is_guest(user_id)  # type: ignore[attr-defined]
+            is_guest = await self.is_guest(user_id)
             if is_guest:
                 return
             is_trial = await self.is_trial_user(user_id)