Make python-gitlab compatible with Python3

This pull request makes gitlab module compatible with Python3. `2to3` tool was used for this purpose and it only replaced `python request` with `python(request)`. Modules used in gitlab module are already ported to Python 3. BTW: print() method was back-ported to Python 2.6 and Python 2.7 (may be other). Thus it could be used in Python 2.x too.