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

Start with the two filter/lambda examples and the PEP 647 reference; compare the requested revealed types and decide which simple predicates are in scope. Done means mypy infers Iterator[int] for the None-check example and Iterator[Tuple[int, int]] for the paired check, with coverage for the supported cases.