nditer.__next__ keeps working after `close` is called

@mattip.

This surprises me a little:
```python
>>> it = np.nditer(np.arange(3))
>>> next(it)
array(0)
>>> it.close()
>>> next(it)
array(1)
>>> next(it)
array(2)
>>> next(it)
StopIteration
```

Compare with
```python
>>> def generator():
...    yield from range(3)

>>> it = generator()
>>> next(it)
0
>>> it.close()
>>> next(it)
StopIteration
```