allow variables named `_` (underscore) to be untyped (throwaway variables)

Feature

There is a very common practice in python, to use the underscore character - _ - as a variable name for a "throwaway character" - mostly when receiving function output values that will not be used, or when implementing functions with defined signatures, without using some of the variables.

The feature request is to allow these variables to be without type hints, and do not warn.

Pitch

In our company, we work a lot with web development, APIs and such, and it is very common to have to implement predefined function signatures. For example, when using AWS lambdas, one needs to implement:
def lambda_handler(event, context):

However, it is very common to only use the event parameter in the code, and not the context - meaning many times we implement
def lambda_handler(event, _):

when typing the function, I currently do something like this

from typing import Any
def lambda_handler(event: dict, _: Any) -> None:

since I do not necessarily know what the _/context variable type is, and I do not care to invest work in finding it when the variable is not in use. It would be easier and nicer if I could simply do this
def lambda_handler(event: dict, _) -> None:

There are plenty of other examples, similar to this, and so I am sure this feature can be useful to many other people. I think even if the feature were to be configurable with a flag (allow_untyped_underscore or something) that would be very helpful as well.

No file, test, or entry point is named. Start by locating mypy's handling of untyped function parameters and existing underscore-related checks; done should mean that an underscore parameter can omit a type hint without a warning, with the intended behavior and any configuration choice clarified.