fix: properties are not required to be writable

[henryiii]

@HDembinski, @jpivarski Important fix to the protocol. The old version required writable properties for axes, kind, and traits. 🤦 . This only requires they be readable properties now. (If they are writable, it does not invalidate the protocol, but it is not required! Not discovered by mplhep usage because the mini-NumPy wrapper happened to have writable properties).

I'd also like to update the protocol to require returning np.ndarray from .values() and such, as you should be a bit more explicit on return types, and less explicit on accepted types. Now that ArrayLike is available, I realize you should only "accept" ArrayLike, and never return it, it's too broad for a return usually.

[henryiii]

The protocol version is now a tuple, to allow minor updates

[henryiii]

This also fixes the enum vs string problem! A sting enum wasn't allowed for .kind, and it turned out it was because .kind was settable; so h.kind = "arbitrary string" needed to be valid. Now that it's a read-only property, it can be a string enum! :)

[henryiii]

Passing in boost-histogram locally, so will merge and make a patch release, as there are no functional changes, just removing functionality that was not supposed to be required.

[HDembinski]

Sounds good to me.