Automatically enable python mode

[Olive <diolu.remove_this_part@bigfoot.com>]

I am using arch GNU/Linux. If a file contains:

#! /usr/bin/python

as the first line, the python mode is automatically enabled. However,
due to the fact that python 2 and python 3 coexists, in order that
the correct python version is reliably involved, the first line must be

#! /usr/bin/python2
or
#! /usr/bin/python3

How can I tell emacs that it have to automatically enable python mode
when one of the two line is the first line?

Olive