MAINT Update six to version 1.4.1

Fixes #2434.
DOC output types should never be "array_like"

We document *exactly* what we give back. "array_like" is the NumPy
way of saying "anything goes that passes np.asarray".
DOC+COSMIT: typos, lots of them

Fixes #2102. There's one typo left in joblib which is better fixed
upstream.
ENH: don't call astype when a copy is not needed

See #2206. This doesn't fix all of the cases.
BUG oops, *safe_* asarray