Pin macOS to 10.15 for wheels - #21303

Doesn't appear sufficient https://github.com/QuLogic/matplotlib/actions/runs/1313592857; this breaks NumPy somehow?

Ah, nope, I just accidentally forgot to update a conditional.

@QuLogic Is this ready to go in?

Looks good now: https://github.com/QuLogic/matplotlib/actions/runs/1314188242

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 v3.4.x
$ git pull

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

$ git cherry-pick -m1 0edfe98f825cbc3f9918e3f615e4dc85478f1f06

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

$ git commit -am 'Backport PR #21303: Pin macOS to 10.15 for wheels'

4. Push to a named branch :

git push YOURFORK v3.4.x:auto-backport-of-pr-21303-on-v3.4.x

5. Create a PR against branch v3.4.x, I would have named this PR:

"Backport PR #21303 on branch v3.4.x (Pin macOS to 10.15 for wheels)"

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!

Remember to remove Still Needs Manual Backport label once the PR gets merged.

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