openpyxl: type to_tree methods

[Avasam]

Overlaps with #9511

One point of interest is that tagname parameter is sometimes passed directly to xml.etree.ElementTree.Element. But itself has the parameter typed as str | Callable[..., Element], which really doesn't make sense to me looking at usages, and there's no comment explaining why. It really seems like the tag parameter of Element.__init__ should not be Callable.

Also, even though Element.itertext does check for possible None self.tag, the docstring and documentation do suggest it should only be a str
Edit: see #10968

That being said, I may need to update some of my tagname parameters to not be possibly None in some/most scenarios.
Done

[github-actions]

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

[srittau]

Spot checking, LGTM