Thank you very much for those tests.  I think you can simplify them a bit. For example, you can use assertRaises.  You also might be able to use the test_support.temp_cwd context manager in your context manager, even though you don't need the cwd part.

I've attached an alternate, simpler patch to fix this bug, based on a similar fix I did in pydoc.  The disadvantage of my patch is that it contains a hardcoding of the name of the function doing the import.  I think this is acceptable given the much greater simplicity of my patch.  I may be missing some subtlety, though, that the twisted folks know about.  Or perhaps people will just find the hardcoding itself objectionable.