fix definition of ndarray.itemset

[dmoisset]

The current definition should probably use ShapeType as an index, and _S as value. To avoid the ambiguity of the 1 argument/2 argument version, an @overload definition could be better.

[dmoisset]

Also, I think itemset makes sense only on ndarray, not on any ArrayLike given that generic is immutable

[abarto-machinalis]

Although it doesn't make sense, itemset IS part of the generic interface. All it does it raise an exception, though.

[dmoisset]

being in the interface but always failing is essentially indistinguishable in python to not being on the interface, so I think it's a good idea to catch this statically if possible