Unpin pytest (pytest-cov's latest release is compatible with it).

[anntzer]

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[jklymak]

LGTM if passes

[jklymak]

I'm going to merge as not a substantive change, just rolling back something that was put in place to deal w/ a prob w/ pytest

[dstansby]

@meeseeksmachine backport to v3.0.x

[dstansby]

@meeseeksdev backport to v3.0.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v3.0.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 4cdc2baa6d0bba96db4b4495ec89d9e05e8c53c3

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am "Backport PR #13141: Unpin pytest (pytest-cov's latest release is compatible with it)."

4. Push to a named branch :

git push YOURFORK v3.0.x:auto-backport-of-pr-13141-on-v3.0.x

5. Create a PR against branch v3.0.x, I would have named this PR:

"Backport PR #13141 on branch v3.0.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.

[timhoffm]

This reverts #13122, which was not backported #13122 (comment). So this needs no backport as well.