explicit python2 and system-site-package for _venv

The explicit use of python2 is usefull on distributions where python3 is the
default python.

The use of system-site-packages was nessesary on my installation. Without
pgi-docgen was unable to import `gi.repository`.
Merge pull request #75 from tvn87/explicit_python2

explicit python2 and system-site-package for _venv