Assertion and checking functions

I'll just leave this here for potential future consideration as it came up on gitter's typing-dev channel. Typescript has something called assertion functions, which enables the type checker to understand that a function does not return if a certain condition is false. I think something similar, but more generalized, would be interesting, especially for unit test functions like assert_is_instance(), assert_true() etc.

def assert_positive(x: object) -> AssertsInstance["x", int]:
    assert isinstance(x, int) and x > 0

Related, checking functions could work like this:

def is_positive(x: object) -> ChecksInstance["x", int]:
    return isinstance(x, int) and x > 0

def my_func(x: Union[int, str]) -> None:
    if is_positive(x):
        # x must be an int
        ...
    else:
        # x can be an int or str
        ...

Comienza con la documentación enlazada de TypeScript 3.7 sobre assertion-functions y con los ejemplos assert_positive e is_positive del issue. Como no se mencionan archivos del repositorio ni pruebas, define primero cómo deben expresar el estrechamiento de tipos las anotaciones generalizadas de aserción y comprobación, y documenta después el comportamiento acordado y la cobertura de pruebas requerida.