run-tests: use a try/except ladder instead of looking for a specific version

This ensures we get json instead of simplejson in as many places as possible.

tests/run-tests.py

@@ -61,10 +61,10 b' from xml.dom import minidom'
 import unittest
 
 try:
-    if sys.version_info < (2, 7):
+    import json
+except ImportError:
+    try:
         import simplejson as json
-    else:
-        import json
 except ImportError:
     json = None