If anything adding a list of assertions at the top of a method/function/class is much more readable than the current type hint implementation.
And if you're running out of resources move the assertions to your regression tests.
Or run Python with -O, which skips assertions.
You know what's even easier? Calling the method. Gives as much useful information in the case of a failure, and also only checks at runtime.
I want something that can check earlier than that, in two ways: before running, and up the call stack.
It doesn't have to work 100% of the time if the 99% of the time it does work it's capable of catching serious errors earlier.