Bug 1696531: add required import to xpcshell/selftest.py r=ahal

Differential Revision: https://phabricator.services.mozilla.com/D109732

testing/xpcshell/selftest.py

@@ -4,7 +4,7 @@
 # http://creativecommons.org/publicdomain/zero/1.0/
 #
 
-from __future__ import absolute_import
+from __future__ import absolute_import, print_function
 
 import mozinfo
 import os