HACKING: tests on python3 require rpm-python3
resolve another use of commands module for python3 compat

03255001d6 moved commands module to src/tito/compat.py to handle
differences between python2 and python3, but missed a bit that
did not show up in unit or functional tests.

Fix those occurences and add unit test to detect the regression.
deprecate GitPython

GitPython is not compatible with python3 and may never be:
https://fedoraproject.org/wiki/User:Churchyard/python3
Merge pull request #109 from jumanjiman/deprecate_gitpython

Deprecate GitPython