[utilities] invert sense of six.PY2 test

Old versions of six do not include a 'PY2' attribute leading to
an exception in sos_get_command_output(). Invert the sense of the
test and check that six.PY3 is false instead as all versions of
the module include this attribute.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[archive] invert sense of six.PY2 test

Old versions of six do not include a 'PY2' attribute leading to
an exception in sos_get_command_output(). Invert the sense of the
test and check that six.PY3 is false instead as all versions of
the module include this attribute.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>