Update Python hashbang to respect environment

The /usr/bin/python hashbang causes doc builds to fail when Python3 is
the default Python and Python2 is being used from within a virtualenv or
Pythonbrew environment.

Signed-off-by: Sharif Olorin <sio@tesser.org>
Merge pull request #1423 from fractalcat/respect-python-env

Update Python hashbang to respect environment