TYP: fix return type of strings.[r]partition - #31912

Unlike np.char.[r]partition, which returns a (single) array, np.strings.[r]partition returns a 3-tuple or arrays:

>>> r = np.strings.partition(np.array(["a b", "c d"]), " ")
>>> type(r), len(r), type(r[0])
(<class 'tuple'>, 3, <class 'numpy.ndarray'>)

The docstring also confirms this.

This fixes the stubs, that previously return (single) arrays here, 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)