codecov no longer uses Windows coverage reports

[pquentin]

We've lost Windows coverage in codecov, as noticed by #77.

It's probably an issue with codecov path merging:

• The coverage reports uploaded by AppVeyor do cover import _winapi at line 14 of _child.py: https://codecov.io/codecov/v4/raw/2020-05-03/CC260CAE9380E25F64740234D25C2B56/8f29cff34e778dca3130400d4e3b5339b0548d9a/875e0775-7e95-433d-bf70-00f09a6d01e6.txt
• But codecov does not see that: https://codecov.io/gh/python-trio/trio-asyncio/src/8f29cff34e778dca3130400d4e3b5339b0548d9a/trio_asyncio/_child.py