dpt.nonzero returns array of int32 dtype #1322

After merging #1300, the result array type of dpt.nonzero is changed:

a = dpt.ones(10, dtype="i8")
dpt.nonzero(a)[0].dtype
# Out: dtype('int32')

The result dtype depends now on array size and may vary between int32 and int64 on the same platform:

sz = numpy.iinfo("i4").max
a = dpt.ones(sz, dtype="i8")
dpt.nonzero(a)[0].dtype
# Out: dtype('int32')

sz = numpy.iinfo("i4").max + 1
a = dpt.ones(sz, dtype="i8")
dpt.nonzero(a)[0].dtype
# Out: dtype('int64')

The python array API states that:

The returned array must have the default array index data type.

and here is about the default array index type:

a default array index data type (either int32 or int64).
...
The default array index data type may be int32 on 32-bit platforms, but the default should be int64 otherwise.