Fix Windows test failure

[haata]

Currently some of the Windows asyncio server tests fail.
https://github.com/capnproto/pycapnp/runs/345170294#step:7:72

This is likely to do with this line https://github.com/capnproto/pycapnp/blob/master/examples/async_server.py#L18

My suspicion is that getTimer() is not returning and is blocking forever. Since this function does work in thread_client.py / thread_server.py I suspect some asyncio detail is starving the capnproto event loop. I've seen similar things happen when integrating pycapnp with PySide2 (Qt5).

It's possible (and likely) that the bug is actually in the test code rather than the pycapnp library.

[haata]

Instead of holding up the v1.0.0 release for another few months, I'm going to skip this test on Windows and add an ssl asyncio calculator test which should check the most common use cases.