use print_function future

This uses `from __future__ import print_function` from "unrestricted" python code.

For restricted python in python scripts, this expects SlapOS to have slapos!1536 .
Running ERP5 with this patch on an old SlapOS software release should probably be OK,
especially because after all we don't have so much critical code using print.

To ease migration of custom project code, PythonCodeUtils was modified so that
when running CodingStyle python scripts are checked with print_function future enabled.

See merge request !1883