package/python-autobahn: drop python 2 support

Upstream commit
https://github.com/crossbario/autobahn-python/commit/c0223223f865603ce192e6d5cdbcf74849a4cdd8,
which has been part of the releases since v20.1.2 has dropped support
for Python 2.x. So python-autobahn is now only available for Python
3.x.

We therefore remove the Python 2.x autobahn tests, and while at it,
drop the python-six dependency, which is no longer needed since
upstream commit
https://github.com/crossbario/autobahn-python/commit/79bd2ba41b83950614a2928eeda4704c1d149a20

,
part of the releases since v20.1.3.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>