Add support for simple lambda type guard

Feature

Consider the following code:

from typing import Optional, List


list_with_none: List[Optional[int]] = [None, 1, 2]


filtered = filter(lambda v: v is not None, list_with_none)
reveal_type(filtered)
# Revealed type is 'typing.Iterator[Union[builtins.int, None]]'

It would be nice if mypy was able to parse simple lambda functions like this one, and infer that filtered is typing.Iterator[builtins.int].

Note that in this example, we use a simple none check, but this could work with more complicated checks:

list_of_tuples: List[Tuple[Optional[int], Optional[int]]]
filter(lambda v: v[0] is not None and v[1] is not None, list_of_tuples)
# would become `Iterator[Tuple[int, int]]`

Pitch

This is a fairly common idiom in Python.

Reasons why mypy might now want to do this:

• Supporting simple lambda is ok, but there will inevitably be a limit to what mypy can support
• We could use https://www.python.org/dev/peps/pep-0647/ to achieve the same thing without using lambdas

Inizia con i due esempi di filter/lambda e il riferimento a PEP 647; confronta i tipi rivelati richiesti e determina quali predicati semplici rientrano nell'ambito. Il lavoro è completato quando mypy inferisce Iterator[int] per l'esempio di controllo di None e Iterator[Tuple[int, int]] per il controllo appaiato, con copertura dei casi supportati.