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Autobahn testsuite is based on Python 2.
See https://github.com/crossbario/autobahn-testsuite/issues/109.

Temporarily use an Ubuntu LTS distro still having Python 2, this will
make the job work for some years.

Hoping in the future to either:
- update Autobahn testsuite to Python 3;
- finding another testsuite for websockets.
Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>