openpyxl: Use duck typing for workbook sheets

[Avasam]

Closes #11706

[JelleZijlstra]

This is an interesting idea, but it feels like something that could well come back to bite us in the future. Any user who starts digging into the types is likely to end up confused.

I'm not completely opposed but I'd like to hear from other maintainers before we commit to this new technique.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Let's try this. I was bitten by this recently when installing types-openpyxl. Maybe this "poor man's AnyOf" (python/typing#566) will help.