Slicing with None adds a new dimension. It's a common paradigm, though usually you'd use A[np.newaxis] or A[np.newaxis, ...] instead for readibility. (np.newaxis is None, but it's a lot more readable)
There's a good argument to be made that slicing with a single None shouldn't add a new axis, and only the more readable forms like A[None, :], A[..., None], etc should.
However, that would rather seriously break backwards compatibility. There's a fair amount of existing code that assumes "A[None]" prepends a new axis.