alias for slice(None)
Having bunch of `slice(None)` in multilevel indexes (>=3 levels) looks pretty unreadable.

Does it make sense to have an alias, maybe `pd.any`?

For example in numpy the `np.newaxis` is alias for `None`:

https://docs.scipy.org/doc/numpy-1.13.0/reference/arrays.indexing.html#numpy.newaxis