Make the install script support python3

[tmadlener]

BEGINRELEASENOTES

• Make ilcsoft-install work with python3. Most of the trivial changes are done via 2to3. Some others that are necessary for this to work with both python2 and python3 are:
  • Fixes to some (relative) imports
  • Importing getoutput and getstatusoutput from the subprocess module if possible and only fallback to the commands module if it does not exist.
  • Replace execfile with exec, compile and open combination.
  • Some changes to the Version class to make it work with the much stricter comparison rules in python3.
• Fix installation of root 6.18/04 by pulling in the patched sources from git directly, instead of using the release tar ball. See: https://root-forum.cern.ch/t/problems-building-root-6-18-04-with-builtin-davix/44225

ENDRELEASENOTES

A few things could still be improved with a probably non-negligible amount of time, e.g.

• Switching to the non-legacy check_output function for getting things from the shell
• Properly introducing logging facilities
• Linting / flake8

[tmadlener]

Should be compatible with python2 and python3:

• Tested with an installation from scratch of v02-02-01, using the scripts/use_gcc82_cvmfs_el7.sh setup script, which runs ilcsoft-install with python2.
• Tested with a head installation based on LCG_99 (see #144), which runs ilcsoft-install with python3