Update guidance for unsigned integer data types for sum and prod

[kgryte]

This PR

• updates the guidance for sum and prod when x has an unsigned integer data type with a smaller range of values than the default integer data type and the dtype kwarg is None. Namely, when providing a uint8 array, the returned array should have a uint32 data type when the default integer data type is int32. This is in contrast to current guidance which requires that the returned array have an int32 data type. In short, the returned array should have a data type whose "signedness" matches the input array. Returning a signed integer array when providing an unsigned integer array is likely to violate user expectations.

The changes introduced in this PR match NumPy's current behavior.

[honno]

LGTM

[kgryte]

Thanks for reviewing, @honno!

Will merge, as this was an initial oversight and aligns with current NumPy behavior and thus with established precedent.