consent fails under py3

[richvdh]

  File "/home/matrix/.synapse-py3/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/home/matrix/synapse/synapse/rest/consent/consent_resource.py", line 148, in _async_render_GET
    self._check_hash(username, userhmac)
  File "/home/matrix/synapse/synapse/rest/consent/consent_resource.py", line 232, in _check_hash
    if not compare_digest(want_mac, userhmac):
builtins.TypeError: a bytes-like object is required, not 'str'

[hawkowl]

fixed by #4140

[hawkowl]

This can be closed when it's deployed to prod and proved working.

[richvdh]

It still failed, but #4163 includes the fix.