Cleanups after we support only Python 3.6 or greater

[facundobatista]

When #411 lands, we can improve code:

• Stop declaring constants in fades/init.py:

# Not using http.server.HTTPStatus to support python < 3.5
HTTP_STATUS_NOT_FOUND = 404
HTTP_STATUS_OK = 200

• There are cleanups in tests/conftest.py (see comments there).