Python 3: Convert some unicode/bytes uses (#3569)

changelog.d/3569.bugfix

synapse/api/auth.py

synapse/federation/transport/server.py

synapse/handlers/auth.py

synapse/handlers/register.py

synapse/http/server.py

synapse/http/servlet.py

synapse/rest/client/v1/admin.py

synapse/rest/client/v2_alpha/register.py

synapse/rest/media/v1/media_storage.py

synapse/state.py

synapse/storage/events.py

synapse/storage/signatures.py

synapse/types.py

synapse/util/frozenutils.py

tests/api/test_auth.py

tests/utils.py