Stubgen: guess return types based on returned values

[yangdanny97]

This PR extends stubgen to guess return types for some functions based on the returned expression. This is inspired by autotyping's scalar-return flag, with the addition of being able to handle literal lists, tuples, sets, and dicts.

There are some limitations:

This only works on concrete functions at least one return statement & no yield/yieldfroms. It only works on literals, tuples of literals, or homogeneous container literals. Additionally, it only works if all the return statements return the same type or None.

To verify this behavior, I added a test case with some examples and updated all the existing stubtest cases.