Fix unittests and add max supported pytest version

[simonfagerholm]

This PR contains 3 changes:

• Fix for running test_subprocess on windows by following instructions on https://docs.python.org/3.7/library/asyncio-subprocess.html#creating-subprocesses
• Change event_loop-fixture to module-scope in hypothesis tests, fixes Hypothesis test fail because of change in Hypothesis #145
• Change setup.py max version of pytest to avoid failures because of changes in 5.4.0 until pytest 5.4.0 deprecated pytest.Function() construction #141 merged. It should be reverted in that PR.

[romainletendart]

Looks good to me.

[simonfagerholm]

@Tinche: Could you have a look at this? It's fixing a problem causing build failures in other PRs

[Tinche]

Thanks!

[simonfagerholm]

Thanks for he merge, @Tinche!