build: only support python3 binary

python2 will be end of life by the time of the next
libvirt release. All our supported build targets, including
CentOS7, have a python3 build available.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

scripts/gensystemtap.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2011-2019 Red Hat, Inc.
 #
@@ -22,8 +22,6 @@
 #   python gensystemtap.py */*.x > libvirt_functions.stp
 #
 
-from __future__ import print_function
-
 import re
 import sys