Use specific commit for pytest and bump pytest-cov

[joguSD]

Some changes to the cli exposed a bug in the stdin/out capturing feature of pytest. I've opened an upstream pull request to fix this and it has been merged: https://github.com/pytest-dev/pytest/pull/1866. Until this version releases next week, I'll be setting the pytest version to be a specific commit including the bugfix to get tests passing again.

@jamesls @kyleknap

[JordonPhillips]

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[kyleknap]

:ship: Might be good to put a todo comment on why you comment out 3.0.1 so that we remember, but if it gets quickly updated this should not be an issue.

[joguSD]

I was actually just about to update that comment to the correct version that the fix should be in.

[kyleknap]

Awesome. :ship: