TYP: fix str_.__getitem__ and bytes_.__getitem__ - #31879

In [1]: np.str_("spam")[0]
Out[1]: 's'

In [2]: np.str_("spam")[0:]
Out[2]: 'spam'

In [3]: np.bytes_(b"spam")[0]
Out[3]: 115

In [4]: np.bytes_(b"spam")[0:]
Out[4]: b'spam'

In [5]: np.str_("spam")[()]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[5], line 1
----> 1 np.str_("spam")[()]

TypeError: string indices must be integers, not 'tuple'

In [6]: np.bytes_(b"spam")[()]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[6], line 1
----> 1 np.bytes_(b"spam")[()]

TypeError: byte indices must be integers or slices, not tuple