If a class, method or function is stub out with an ellipsis (...), black gives them their own line in python files and appends them end-of-line after the colon in stub files:

scratch.py

class Animal:
    ...


class Cat(Animal):
    def meow(self):
        ...


def f():
    ...

scratch.pyi

class Animal: ...

class Cat(Animal):
    def meow(self): ...

def f(): ...