TYP: fix output dtype of bincount with weights - #31911

If weights are passed to bincount, the returned dtype is float64, not intp:

>>> np.bincount(np.array([0, 1, 1, 2]), weights=np.array([.5, 1, 1.5, 2])).dtype
dtype('float64')
>>> np.bincount(np.array([0, 1, 1, 2]), weights=np.array([1, 2, 3, 4])).dtype
dtype('float64')

This fixes the stubs to match this runtime behavior

Fully written by me (a human); bug found by Claude Fable 5 (here's the full (second) audit for those interested).

(more stub fixes incoming, btw)