Add __init__ definition for AttributeError

[klaseskilson]

Since Python 3.10, AttributeError supports keyword arguments for name and obj.

Because of this, doing something like this will cause issues with Mypy:

def do_something(something):
    if not something['field']:
        # Mypy gives you `error: Unexpected keyword argument "obj" for "AttributeError" [call-arg]`
        raise AttributeError(obj=something, name='field')
    ...

Seems like this was only due to a missing __init__ definition.

---

First contribution here. Sorry if I missed a step or made some mistake.

[AlexWaygood]

Looks good! Just one small thing to fix :)

[AlexWaygood]

Thank you! And welcome to typeshed!

[github-actions]

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