Declaring classes as pure mixins

(This is an extension to #241 and copied from the discussion there.)

What about having a magic class decorator (with no or minimal runtime effect) or a base class that declares a class as a "pure mixin"? A pure mixin could only be used for implementation inheritance, and it wouldn't be type checked by itself at all -- it would only be type checked as part of implementation inheritance. I think that this would allow type checkers to check some mixin use cases that are tricky right now with minimal code changes. The alternative would be to use ABCs to declare the things that the mixin expects to be defined elsewhere, but they are harder to use and arguably add boilerplate.

Example:

@puremixin
class MyMixin:
    def do_stuff(self) -> int:
        self.one_thing()  # ok, even though not defined in this class
        return self.second_thing(1)  # also ok

class Thing(Implementation[MyMixin]):
    def one_thing(self) -> None: pass  # ok
    def second_thing(self, x: int) -> str:    # error: return type not compatible with do_stuff
        return 'x'

Esta es una propuesta de diseño copiada de la discusión en #241 y no menciona archivos ni pruebas. Empieza leyendo #241 y su discusión y, a continuación, aclara la semántica de un decorador o una clase base de mixin puro y cómo debería comprobarse el caso mostrado de Implementation[MyMixin]; el trabajo se considerará terminado cuando haya un diseño acordado.