minor adjustment of safeinput unit test to make lint happy on py3 and stop corresponding github action failing

git-svn-id: svn+ssh://svn.code.sf.net/p/migrid/code/trunk@6145 b75ad72c-e7d7-11dd-a971-7dbc132099af

Author: jonasbardino

tests/test_mig_shared_safeinput.py

@@ -31,7 +31,7 @@
 import importlib
 import os
 import sys
-from past.builtins import basestring
+from past.builtins import basestring, unicode
 
 from tests.support import MigTestCase, testmain