docker: drop python2

Re-apply patch Iacace670bdb949e67227e0faa1765e36bba42018, after
providing additional patches to make all Osmocom repositories, that use
external tests, work with python3 (after learning the hard way, that
they were not really python3 compatible yet, although they looked like
it):

https://gerrit.osmocom.org/q/topic:drop-py2

This patch should not be merged, before all other patches from that
topic are merged.

Depends: osmo-python-tests I3ffc3519bf6c22536a49dad7a966188ddad351a7
Related: OS#2819
Change-Id: I7b5385416d5c48a2f4e07449192b3f74223cb094
1 file changed