Narrowing types to `Literal` using `in` syntax

Take this example code:

x = "a"
reveal_type(x)
assert x in ("a", "b", "c")
reveal_type(x)

y = 20
reveal_type(y)
assert y == 10 or y == 20 or y == 50
reveal_type(y)

The current output of this is:

$ mypy x.py 
x.py:2: note: Revealed type is "builtins.str"
x.py:4: note: Revealed type is "builtins.str"
x.py:7: note: Revealed type is "builtins.int"
x.py:9: note: Revealed type is "builtins.int"

If mypy were to support type narrowing using chained or statements and in statements, the output would look like:

$ mypy x.py 
x.py:4: note: Revealed type is "builtins.str"
x.py:6: note: Revealed type is "Union[Literal['a'], Literal['b'], Literal['c']]"
x.py:9: note: Revealed type is "builtins.int"
x.py:11: note: Revealed type is "Union[Literal[10], Literal[20], Literal[50]]"

I think doing simple narrowing here would be relatively easy and very useful.

Inizia con gli esempi x.py nell'issue ed esegui mypy per confermare i tipi attualmente rivelati. Analizza il comportamento esistente di mypy nel restringimento dei tipi per condizioni or concatenate ed espressioni in; il lavoro è completato quando gli esempi rivelano le unioni Literal richieste sia per i casi di stringhe sia per quelli di interi.