BUG: einsum rejects spaces around ellipsis:

```python
>>> x = np.eye(1)
>>> np.einsum('...ab, ...bc ->...ac', x, x)
np.array([[1.]])
>>> np.einsum('...ab, ...bc -> ...ac', x, x)
ValueError: einstein sum subscripts string contains a '.' that is not part of an ellipsis ('...')
```