These things seemed to be completely untested, so I added a load of tests for
them.

This also removes some duplicated code between the simple
HTTP client and matrix federation client.

This also expands type-hints to the SSO and registration code.

Refactors the CAS code to more closely match OIDC/SAML.

Fixes #4042

This also fixes a bug by fixing handling of an account which doesn't expire.

Optionally sends typing, presence, and read receipt information to appservices.

This seemed to entail dragging in a type stub for SortedList.

This requires adding a mypy plugin to fiddle with the type signatures a bit.

* update version of black and also fix the mypy config being overridden