Version 1.5.4

• Bugfix so that the return value of JWTAuth.refresh() correctly matches
  that of the auth interface (by returning a tuple of
  ((access token), (refresh token or None)), instead of just the access token).
  In particular, this fixes an exception in BoxSession that always occurred
  when it tried to refresh any JWTAuth object.
• Fixed an exception that was being raised from ExtendableEnumMeta.__dir__().
• CPython 3.6 support.