avoid setting explicit "None" as StopIteration exception value in generators and directly return NULL instead

disable unicode default encoding in pypy: PyBytes_AsStringAndSize() does not handle Unicode default encoding in pypy and _PyUnicode_AsDefaultEncodedString() does not return a cached borrowed reference but a new owned one

mofe GIVEREF() before PyTuple_SET_ITEM() call because that's when we still own the reference (pypy needs it that way)