Point incompatible assignment errors to rvalue instead of lvalue

[JukkaL]

This seems more logical. If there is a tuple lvalue, we point to the
incompatible tuple item.

Note that this may break some # type: ignore statements since
the reported line number may change (but it usually doesn't).

[ilevkivskyi]

Thanks, makes sense! Do we need a similar change to augmented assignments like x += y? (Maybe self.msg.incompatible_operator_assignment(s.op, s) call in visit_operator_assignment_stmt` needs to be updated.)

[JukkaL]

Do we need a similar change to augmented assignments

They work like this already.