Needs to be fixed upstream (by werkzeug, twisted). Doesn't impact the functionality and the tests are soon to be removed with the py3 update anyway.
cmp is deprecated since attrs 19.2.0: http://www.attrs.org/en/19.2.0/changelog.html The deprecation warning breaks the doctests. Fortunately they have a rather long deprecation window, so we can just wait until upstream(s) fix this.