As discussed a while ago, switch the utils (including their tests) to
use python3 by default. While on it, drop usage of "env" to always get
the system python3 instead of a random one that happens to live
somewhere in $PATH.

In practise, this patch doesn't change much - AFAIK openSUSE, Debian and
Ubuntu already patch aa-* to use python3.

Also add a note to README to officially deprecate Python 2.x.
(I won't break Python 2.x support intentionally - unless some future
change gives me a very good reason to finally drop Python 2.x support.)



Acked-by: Seth Arnold <seth.arnold@canonical.com>
(since 2016-08-23, but the commit had to wait for the FileRule series
 because it touches test-file.py)