Set sys.modules[name] to None instead of 0 to block module import.

Lib/test/support.py

@@ -109,7 +109,7 @@ def _save_and_block_module(name, orig_modules):
         orig_modules[name] = sys.modules[name]
     except KeyError:
         saved = False
-    sys.modules[name] = 0
+    sys.modules[name] = None
     return saved