Just to note, that there are two – somewhat distinct – issues here in my opinion:

1. `FromSpec` does not scan `bases` for the correct metaclass, which it could; this could even be considered a bug?
2. You cannot pass in a desired metaclass, which may require a new API function.

My patch tries to address the first (the class creator has to take care that this is reasonable for the metaclass).  I had hoped the `slot` mechanism can avoid the API discussion for the second one, but I guess not.


On the discussion on `tp_type/meta` being incorrect usage:
I am slightly surprised we actually care about static C-definitions?

There is no reason that a `spec` should be declared static (aside that you have to move it into the function otherwise)?  Everything is copied by `_FromSpec` after all.
However, I suppose that would replace a safe-by-design API with a "best practice" to never define the spec/slots statically (a best practice that is probably not generally followed or even advertised currently, I guess).