CI: Add pytest-4 and Python 3.8 tests

[marv]

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Would be great if CI could tests these combinations as well as more distros use pytest 4 (or even 5 already) and python 3.8

[blueyed]

Tests are not compatible with newer pytest versions. See https://github.com/pytest-dev/py/issues/104, and https://github.com/pytest-dev/py/pull/222 (and likely more).

[marv]

Tests are not compatible with newer pytest versions. See #104, and #222 (and likely more).

Yes, those issues brought me here, I'm working on updating pytest to version 4 on Exherbo and ran into those issues. I thought it would be helpful to have CI test those environments when trying to fix those issues. But I see that they would need to be marked "allowed to fail" for now, so that they don't cause the tests to fail