Fixup synapse.rest to pass mypy (#6732)

changelog.d/6732.misc

mypy.ini

synapse/rest/admin/users.py

synapse/rest/client/v1/login.py

synapse/rest/client/v1/room.py

synapse/rest/client/v2_alpha/register.py

synapse/rest/client/v2_alpha/sendtodevice.py

synapse/rest/key/v2/remote_key_resource.py

synapse/rest/media/v1/media_repository.py

synapse/rest/media/v1/preview_url_resource.py

synapse/rest/media/v1/thumbnail_resource.py

tox.ini