ci: use python3 in linux-gcc and osx-gcc and python2 elsewhere

ci: use python3 in linux-gcc and osx-gcc and python2 elsewhere

Python2 reached end of life, and we have been preparing our Python
scripts to work with Python3.  'git p4', the main in-tree user of
Python, has just received a number of compatibility updates.  Our
other notable Python script 'contrib/svn-fe/svnrdump_sim.py' is only
used in 't9020-remote-svn.sh', and is apparently already compatible
with both Python2 and 3.

Our CI jobs currently only use Python2.  We want to make sure that
these Python scripts do indeed work with Python3, and we also want to
make sure that these scripts keep working with Python2 as well, for
the sake of some older LTS/Enterprise setups.

Therefore, pick two jobs and use Python3 there, while leaving other
jobs to still stick to Python2 for now.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>