BUG: Fix bug in putmask for CoW

[phofl]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

cc @jorisvandenbossche this did not work an 1.5.2

[phofl]

cc @jorisvandenbossche Could you have a quick look? Ideally this would go into 1.5.3

[mroeschke]

LGTM

[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 1.5.x
git pull

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

git cherry-pick -x -m1 954cf55938c50e34ddeced408d04bd04651eb96a

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

git commit -am 'Backport PR #50630: BUG: Fix bug in putmask for CoW'

4. Push to a named branch:

git push YOURFORK 1.5.x:auto-backport-of-pr-50630-on-1.5.x

5. Create a PR against branch 1.5.x, I would have named this PR:

"Backport PR #50630 on branch 1.5.x (BUG: Fix bug in putmask for CoW)"

And apply the correct labels and milestones.

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

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

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

[jorisvandenbossche]

Should we backport #50675 as well?

[jorisvandenbossche]

Ah, forget what I said, the usage of using_copy_on_write is in the internals itself, so there it is fine as is.

[phofl]

Puuuuh, is there anything else that is scheduled for 1.5.3 regarding CoW? If not, I wouldn't backport I think

[phofl]

Good :)