TYP: Fix flatiter.__next__ return type for object_ and StringDType - #31660

Do you want to care about the fact that accessing a StringDType array can return the na_object as well?

>>> import numpy as np
>>> flt = np.array(["x", None], dtype=np.dtypes.StringDType(na_object=None)).flat
>>> flt[0]
'x'
>>> flt[1]
>>> flt[1] is None
True

It looks like this pattern occurs elsewhere (ndarray.item, ndarray.__iter__, ndarray.tolist, and flatiter.__getitem__(int) to name a few) so feel free to disregard this for now and do a fix focused on that later.

Do you want to care about the fact that accessing a StringDType array can return the na_object as well?

Part of me does, but I'm also a bit worried that doing so will set a precedent where we'll have to add special-casing for this all over the place. So I'm not sure if that's the way we want to go, given that the stubs are already a big mess.
But on the other hand, the stubs are already a big mess, so the ∂mess/∂code probably won't be very high.

In the stubs StringDType is already generic on the na object type, so at least that part is already possible: