Constrain/narrow literal type with 'in' checks

```py
from typing import Literal

a: Literal[1, 2, 3] = 2
assert a in (1, 2)
b: Literal[1, 2] = a
```
```
error: Incompatible types in assignment (expression has type "Union[Literal[1], Literal[2], Literal[3]]", variable has 
type "Union[Literal[1], Literal[2]]")
    b: Literal[1, 2] = a
                       ^
```