[CRIU] Add support for Python2/Python3

[Adrian Reber]

This patchset changes crit to support both python2 and python3.

This is the minimal changeset which would allow me to build CRIU
packages for Fedora using python3 and for CentOS using python2.

crit is the only python script installed via packages and therefore
this would be enough for my use case in the first step.

coredump and zdtm.py are still python2 only.

My changes detect if python2 or python3 is available with python2
as a preference (for now). So for existing users nothing should
change. For python3 only systems it should also just work with
these changes.

		Adrian