add py38 and py35 to the appveyor matrix

[bollwyvl]

Further investigates whether the current release meets the need on python 3.8 on windows.
Also changes one of the 3.6 excursions to 3.5. The setup.py claims 3.4, but meh, and ensures the (apparently) broken pytest 5.3.4 is not used...

see also:

• Merge pull request #52 from regro-cf-autotick-bot/5.1.4 conda-forge/ipykernel-feedstock#52
• 5.1.4, noarch conda-forge/ipykernel-feedstock#51

[bollwyvl]

Well, looks like I broke appveyor for some hours again...

[Carreau]

restarted the failing build.

[Carreau]

Seem like it would need #480 for tests to pass.

[bollwyvl]

I think there's still more work to be done in the test suite to apply the asyncio patch... I went heavy on the conda-forge feedstock, and just applied the patch before calling pytest.main in a wrapper script, which seemed a bit dirty.

…

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[Carreau]

Well seem to be working now;