Rev 45706 renamed stuff in contextlib.py, but didn't rename

Rev 45706 renamed stuff in contextlib.py, but didn't rename
uses of it in test_with.py.  As a result, test_with has been skipped
(due to failing imports) on all buildbot boxes since.  Alas, that's
not a test failure -- you have to pay attention to the

    1 skip unexpected on PLATFORM:
        test_with

kinds of output at the ends of test runs to notice that this got
broken.

It's likely that more renaming in test_with.py would be desirable.