Port over enough to get some sytests running on Python 3 (#3668)

changelog.d/3668.misc

synapse/api/auth.py

synapse/api/ratelimiting.py

synapse/config/logger.py

synapse/http/servlet.py

synapse/http/site.py

synapse/rest/client/transactions.py

synapse/rest/media/v1/upload_resource.py

synapse/secrets.py

synapse/util/logutils.py

synapse/util/stringutils.py

synapse/util/versionstring.py