Can't create dtype without any casting implementations

[rossbar]

... not that you should be able to.

I only mention this because at some point I think you had mentioned that you only wanted to require a self-cast impl for parametric dtypes, but the following block seems to make the requirement apply to all dtypes

https://github.com/seberg/numpy/blob/e9ce43885c6c5cf3fd91803796afec13e69f2ff5/numpy/core/src/multiarray/experimental_public_dtype_api.c#L310-L321

From the comment there it seems like you're already on top of this, but I wanted to bring it up as I hit this branch when trying rationals.