python: Don't hardcode interpreter path

python: Don't hardcode interpreter path

This is particularly useful on operating systems that don't ship
Python as part of the base system (eg. FreeBSD) while still working
just as well as it did before on Linux.

While at it, make it explicit that our scripts are only going to
work with Python 2, and remove the usage of unbuffered I/O, which
as far as I can tell has no effect on the output files.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>