a test for an error condition not covered by existing tests

(noticed this when writing the equivalent code for pypy)

Lib/test/test_descr.py

@@ -3622,6 +3622,13 @@ def test_mutable_bases():
     else:
         raise TestFailed, "shouldn't be able to assign to list.__bases__"
 
+    try:
+        D.__bases__ = (C2, list)
+    except TypeError:
+        pass
+    else:
+        assert 0, "best_base calculation found wanting"
+
     try:
         del D.__bases__
     except TypeError: