Python shebang

Currently, in the interregnum where Python 2 and Python 3 may co-exist on the same system, the PEP 0394 recommendations for the shebang line in Python programs run in short like this:

1. Use #!/usr/bin/env before Python interpreter
2. Use #!/usr/bin/env python only for programs that work with both Python 2 and Python 3
3. If your program runs with Python 3 only, replace python in the previous line with python3
4. Else, for Python 2-based programs, replace python with python2