ENH: Create string dtype instances from the abstract dtype

[ngoldbaum]

Following up from #22863 (comment), this makes it possible to create string dtype instances from an abstract string DTypeMeta.

Should I support any additional arguments or keywords to __new__ besides size? Possibly byteorder?

Should this be documented somewhere? If so, where?

[seberg]

Thanks, a few comments, but generally I am happy with adding this.

Documenting this seems awkward (even the release note...) until we have a clear spelling for type(np.dtype('U')).
After that, it seems pretty obvious to document it in the class.

[ngoldbaum]

Thanks for the comments! I used npy_mul_sizes_with_overflow to deal with the overflow based on other examples in numpy. Let me know if you spot any issues with my usage.

[seberg]

Happy, although that swap would be nice (adding a test would also be nice maybe, since 2**30 hits it).

I will assume nobody minds this tiny API addition, so planning to merge soon.

[seberg]

Changed this to value error, because I thought even the other ones make more sense that way. But then, it shares the error with other paths, so only did this one.

If anyone doesn't like, happy to follow up. Will merge once tests are done, thanks Nathan.