Make build and external tests work with python3, so we can drop
the python2 dependency.

This should be merged shortly after osmo-python-tests was migrated to
python3, and the jenkins build slaves were (automatically) updated to
have the new osmo-python-tests installed.

Related: OS#2819
Depends: osmo-python-tests I3ffc3519bf6c22536a49dad7a966188ddad351a7
Change-Id: I48f4c2c520e8285aff5d6d65f95bd041c13466e8