Ensure that astype converts input to marray

[mdhaber]

Closes gh-161.

[lucascolley]

CI is rather unhappy

[mdhaber]

I see. It has to do with data-apis/array-api#1009. I'd appreciate your thoughts there.

It's a royal pain if the decision is that astype with copy=False must return the input object but asarray can create a new object. This would mean that astype can't easily use asarray for input standardization. And the whole point of this issue is that MArray tries to be flexible when the input is not already an MArray (but is instead an array of the backend library).