Bug 1567642 - [mozharness] Fix missed print statement in 'test_base_script.py', r=Callek

Differential Revision: https://phabricator.services.mozilla.com/D45666

--HG--
extra : moz-landing-system : lando

testing/mozharness/test/test_base_script.py

@@ -56,7 +56,7 @@ def get_debug_script_obj():
 
 def _post_fatal(self, **kwargs):
     fh = open('tmpfile_stdout', 'w')
-    print >>fh, test_string
+    print(test_string, file=fh)
     fh.close()