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The caching mechanism is going to check for changes in the directory. So the recommendation is to clear the cache manually:
<quote>
 The default finders used by import now utilize a cache of what is contained
 within a specific directory. If you create a Python source file or sourceless
 bytecode file, make sure to call importlib.invalidate_caches() to clear out
 the cache for the finders to notice the new file.
</quote>
(see http://docs.python.org/dev/whatsnew/3.3.html#porting-python-code)
That section doesn't really talk about the directory side of things, but calling importlib.invalidate_caches() works great to eliminate the error in your example.