Changes

configure.ac: migrate from python2 to python3

VTY and CTRL tests pass fine with python3. Drop the python2 requirement,
so we can use debian 12 in CI. Some of the files in openbsc/contrib
probably still need python2, but since this is a legacy project we
probably don't care.

Related: OS#5950
Change-Id: I052c59dcc21b8e1dd4a3460cf8af9ccbeed6de5b