Cleaner usage of get_proper_type() (#13326)
This is a follow up for #13297
I move around some calls to `get_proper_type()` to preserve original types as much as possible (plus few related required low-risk changes). This makes error messages much more concise, before some error messages in the tests I added were truly epic:
```
Incompatible types in assignment (expression has type "Sequence[Union[B, Sequence[Union[B, Sequence[Union[B, Sequence[Union[B, Sequence[Union[B, Sequence[Union[B, NestedB]]]]]]]]]]]]", variable has type "int")
```
