selftests/functional/test_thirdparty_bugs.py: use six for compatiblity

Instead of trying locations on Python 2 and 3.  The same pattern is
used on other locations.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/functional/test_thirdparty_bugs.py

@@ -2,10 +2,7 @@ import re
 import json
 import unittest
 
-try:
-    from urllib2 import URLError
-except ImportError:
-    from urllib.error import URLError
+from six.moves.urllib.error import URLError
 
 from avocado.utils import download