They are originally magic comments, unformalized. The feature is originally known as annotations and you can basically put anything there.
def f(a: "something like an int", b: "another int-like thing"): pass
def g(a: sys.stdin, b: sys.stderr): pass
You can run a third party linter on those comments, but you must hope that they're correct. There are usually some checks for that, but they're only reliable in trivial cases.
This is not static typing any more than "you can use emscripten to transpile JavaScript to C" means that JavaScript is a low level language with native assembly support. It's a huge step forward from "no system at all" and I'm thrilled it exists, but it's hardly the same thing.