migrate to python3
Python 2 is scheduked for complete deprecation by upstream in 2 years - https://pythonclock.org/
We should migrate all remaining python 2 code to python 3.
The code in osmo-python-tests iscompatible with both python 2 and 3 as of 2f266e0cec10e32db8781a8fb788b0432c33d6d9.
This allows to migrate code which uses it (ctrl and vty tests) to python 3. Once there're no more python 2 code depending on osmo-python-tests, we can drop support for python 2 from osmo-python-tests.