notpython2.py

#!/usr/bin/env python3

from __future__ import print_function

import sys

if sys.version_info[0] < 3:
    raise RuntimeError("This program requires Python 3.x")
    # or to avoid having a traceback:
    #print("This program requires Python 3.x", file=sys.stderr)
    #sys.exit(1)

def main():
    print("Hello World!")

if __name__ == '__main__':
    main()