REF: values-> _values
For the affected Index subclasses .values and ._values are the same
@WillAyd tangential to this PR: in `io.json._json `there are a couple of uses of `obj.values`; is it clear whether those can be changed to `obj._values`? I don't know all of the differences between the two enough to say for sure but I would think so > I don't know all of the differences between the two enough to say for sure but I would think so

Yah it can be hard to keep track of.  Rule of thumb: `._values` is non-lossy and non-copying, `.values` may doesn't assure either for EA Series/Index.

I'll try to changing those two usages and see if it breaks anything.