Port rest/ to Python 3 (#3823)

changelog.d/3823.misc

synapse/rest/client/v1/admin.py

synapse/rest/client/v1/events.py

synapse/rest/client/v1/initial_sync.py

synapse/rest/client/v1/login.py

synapse/rest/client/v1/push_rule.py

synapse/rest/client/v1/pusher.py

synapse/rest/client/v1/room.py

synapse/rest/client/v2_alpha/sync.py

synapse/rest/client/v2_alpha/thirdparty.py

synapse/rest/key/v1/server_key_resource.py

synapse/rest/key/v2/__init__.py

synapse/rest/key/v2/remote_key_resource.py

synapse/rest/media/v0/content_repository.py

synapse/rest/media/v1/_base.py

synapse/rest/media/v1/download_resource.py

synapse/rest/media/v1/media_repository.py

synapse/rest/media/v1/preview_url_resource.py