And the compatiblity helper six.  With Python 2 gone, a number of
package changes are also introduced, so that the packages are always
named python3-<suffix>.

Also, because it would be confusing not having an "avocado" script,
and to conform with the system wide change introduced by distros that
now offer Python 3 by default, the scripts are no longer called
avocado-3 (or avocado-3.x), but simply "avocado".
Signed-off-by: NCleber Rosa <crosa@redhat.com>