Avoid false "Incompatible return value type" errors when dealing with `isinstance`, constrained type variables and multiple inheritance (Fixes #13800)

[tyralla]

Avoid false "Incompatible return value type" errors when dealing with isinstance, constrained type variables and multiple inheritance (Fixes #13800).

The idea is to make a union of the current expanded return type and the previously expanded return types if the latter are among the bases of intersections generated by isinstance checks.

[github-actions[bot]]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[tyralla]

@hauntsaninja: I could not resist checking if I could solve the issue relatively cleanly. Mypy's multi-pass approach for constrained type variables makes my solution more "global" than I hoped (I mean the addition of a new TypeChecker attribute). Still, it is not too invasive, in my opinion. Would you like to review it?