Disable io.ascii multiprocessing on Windows for now

As @embray noted in #2952, some io.ascii tests fail on Windows because there is a problem with Windows multiprocessing. This PR temporarily disables multiprocessing on Windows (and contains a minor segfault fix).
I think instead of silently skipping the test I'd rather explicitly call `pytest.xfail('...reason...')` so that we know it should work sometime but is currently expected to fail.  Thanks!