`AbstractContextManger.__enter__` should be abstract

Code sample in basedpyright playground

from contextlib import AbstractContextManager
from typing import override


class CM(AbstractContextManager[int]):
    @override
    def __exit__(self, *_): ...

with CM() as i:
    print(i + 1)

AbstractContextManger.__enter__ has a default implementation that returns self, so by making it abstract you would require implementations to provide the correct implementation. this would force a redundant implementation if the type is Self, but that is currently impossible to type anyway: https://discuss.python.org/t/self-as-typevar-default/90939

even without Self support, it is only one line of redundant code: def __enter__(self) -> Self: return self

Start with the basedpyright playground example and the AbstractContextManager stub entry point it exercises. Check how the current default enter affects abstract-class checking, then verify that making it abstract requires an implementation while preserving the intended context-manager typing behavior.