Inconsistent behavior of "same_kind" casting wrt. signed/unsigned ints

[anntzer]

Initially noticed in #8034: "same_kind" casting refuses to drop a minus sign when casting signed int -> unsigned int, but happily wraps around large values to negative values when casting unsigned int -> signed int:

In [17]: np.array(-1, np.byte).astype(np.ubyte, casting="same_kind")
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-17-100f7edb5d2f> in <module>()
----> 1 np.array(-1, np.byte).astype(np.ubyte, casting="same_kind")

TypeError: Cannot cast array from dtype('int8') to dtype('uint8') according to the rule 'same_kind'

In [18]: np.array(129, np.ubyte).astype(np.byte, casting="same_kind")
Out[18]: array(-127, dtype=int8)

(This is also affects casting of arrays: their values are specifically checked.)
I personally think the latter behavior (wraparound allowed) should be used in both cases... but at least it should be consistent :-)