Allow heterogeneous concatenation for list

[tamuhey]

From microsoft/pyright#1777

[0] + ["a"] is not allowed in any type checker (e.g. pyright, mypy) because the type annotation of list.__add__ is defined as follows:

class list(MutableSequence[_T], Generic[_T]):
    ...
    def __add__(self, x: List[_T]) -> List[_T]: ...

This annotation prohibits heterogeneous concatenation, although it is actually allowed in runtime.
I think the annotation should be as follows to solve this problem:

class list(MutableSequence[_T], Generic[_T]):
    ...
    def __add__(self, x: List[_S]) -> List[Union[_T, _S]]: ...

[JelleZijlstra]

See #3183 for some past attempts.

[tamuhey]

Also related: #2383