openpyxl: tag attr can be callable

[Avasam]

As noticed in #11841 (comment)

The tag can actually be a str OR a callable.
The only place the tag attribute isn't for a comparison (equal, contains, itemgetter, ...) is: https://foss.heptapod.net/openpyxl/openpyxl/-/blob/700cafe4928c9c5e7ffc998f5d911e475ac8c37f/openpyxl/descriptors/serialisable.py#L76
And that's after being passed through the localname function [where it simply checks if it's a callable]: https://foss.heptapod.net/openpyxl/openpyxl/-/blob/700cafe4928c9c5e7ffc998f5d911e475ac8c37f/openpyxl/xml/functions.py#L77-81

[github-actions]

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

[Avasam]

Right hum... I don't think there's a clean or easy way to do what I'm trying to here. So maybe just keeping the status-quo for now.