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

@@ -61,12 +61,12 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
+    except ImportError:
+        json = None
 
 processlock = threading.Lock()