Update to 21.3.1 (debian/unstable branch)

This MR updates the packaging of python-autobahn from 0.10.3+dfsg1 to 21.3.1. Closes #994914.

Some lintian warnings remain. I lack knowledge to resolve them, however:

1. excessive-debhelper-overrides: not sure how as I do not see any lintian overrides in debian/, maybe there are openstack-wide settings?
2. typo-in-debhelper-override-target override_dh_gconf => override_dh_icons: seems false-positive to me, but I may be wrong.
3. unusual-interpreter /usr/bin/python3.9: I suspect this is due to python-setuptools adding the interpreter while producing CLI commands.