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
        ...

Beginne mit der verlinkten Dokumentation zu den TypeScript 3.7 assertion-functions und den Beispielen assert_positive und is_positive aus dem Issue. Da keine Repository-Dateien oder Tests genannt sind, definiere zunächst, wie generalisierte Assertion- und Prüfannotationen die Typverengung ausdrücken sollen, und dokumentiere anschließend das vereinbarte Verhalten und die erforderliche Testabdeckung.