asyncio: make AbstractServer abstract and remove unnecessary metaclass=ABCMeta

[AlexWaygood]

I've been fiddling about with a script to identify functions that:

1. from an AST analysis of the runtime source code, appear to return None at runtime.
2. Are annotated as returning something other than None at runtime.

One thing the script has identified is that there seem to be a bunch of classes in asyncio that are not marked as being abstract in typeshed, but look awfully like they're intended to be abstract at runtime.

[Akuli]

$ grep -r metaclass=ABCMeta stdlib/asyncio/
stdlib/asyncio/events.pyi:class BaseDefaultEventLoopPolicy(AbstractEventLoopPolicy, metaclass=ABCMeta):
stdlib/asyncio/base_events.pyi:class BaseEventLoop(AbstractEventLoop, metaclass=ABCMeta):

Neither of these seems to be necessary.

[AlexWaygood]

$ grep -r metaclass=ABCMeta stdlib/asyncio/
stdlib/asyncio/events.pyi:class BaseDefaultEventLoopPolicy(AbstractEventLoopPolicy, metaclass=ABCMeta):
stdlib/asyncio/base_events.pyi:class BaseEventLoop(AbstractEventLoop, metaclass=ABCMeta):

Neither of these seems to be necessary.

I think the metaclass lie is actually necessary for both of these classes, unfortunately. If we do not lie about the metaclass, then mypy will complain about not all abstractmethods being implemented -- because there are no abstractmethods defined in the body of either of these classes, mypy will assume that the classes are meant to be a concrete implementation of the abstract classes they inherit from.

The way to get mypy to accept that the subclass is also meant to be abstract, even though it defines no abstractmethods itself, is explicitly re-specify the metaclass in the subclass. Unfortunately, in this case that means lying about the metaclass.

But, let me check...

[Akuli]

tests/mypy_test.py passes to me with metaclass=ABCMeta removed from BaseEventLoop, although the one in BaseDefaultEventLoopPolicy seems to be necessary.

[AlexWaygood]

tests/mypy_test.py passes to me with metaclass=ABCMeta removed from BaseEventLoop, although the one in BaseDefaultEventLoopPolicy seems to be necessary.

Okay, let's try it

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉