ENH: add warning when calling ufunc with 'where' and without 'out'

[mattip]

Closes #29804, related to # #17192, #29561 and maybe more

Users naively add a where argument without a proper out argument, and are surprised when the False values of the where mask are uninitialized. This PR adds a warning. The warning can be surpressed by using where=mask, out=None. That came in handy in some obscure tests.

[seberg]

Thanks! I assume this is rare enough that introducing a warning is not a problem and it is very simple to remove the warning after all.

So I am in favor of trying this, approving, but the comments should be (mostly) addressed.

Test changes look good, the only ones I would worry about would be __array_wrap__ related, and those don't seem touched.

[mhvk]

Yes, nice; we really should have done this earlier...

[mhvk]

Nice! (I think my last comment was wrong, though)

[andyfaff]

Is it possible to add a stacklevel value to the warning, because it'd be nice to see the warnings caller.

[seberg]

The warning already has the best possible stacklevel since it's in C (so no need to add one). But of course sometimes ufuncs are called indirectly (e.g. mean/var maybe).

I am not sure if there is a nice way to add skip_file_prefixes in C, although I guess one could bother to just call into Python to do it (warnings are slow anyway, I guess).
(This would also have to be Python 3.12 only as it is a new feature)