Type hints for tests.appservice (#14990)

* Accept a Sequence of events in synapse.appservice

This avoids some casts/ignores in the tests I'm about to fixup. It seems
that `List[Mock]` is not a subtype of `List[EventBase]`, but
`Sequence[Mock]` is a subtype of `Sequence[EventBase]`. So presumably
`Mock` is considered a subtype of anything, much like `Any`.

* make tests.appservice.test_scheduler pass mypy

* Extra hints in tests.appservice.test_scheduler

* Extra hints in tests.appservice.test_api

* Extra hints in tests.appservice.test_appservice

* Disallow untyped defs

* Changelog