Relax python file magic regex somewhat

Relax python file magic regex somewhat

- libmagic sometimes adds trailing descriptions about encodings, line
  terminators and whatnot, we dont want to care about those (RhBug:796218)
- not all python-related strings start with [pP]ython either, sometimes
  libmagic says "a python script" or "a /usr/bin/python script" and
  whatnot, so loose the start-of-line restriction as well