Remove `function` from builtins

[davidhalter]

There's a class function in builtins.pyi. There's also a mention to remove it: # TODO not defined in builtins!. I feel like this was probably created for mypy? As an alternative I would suggest using types.pyi's FunctionType.

I personally have to patch typeshed to remove this for Jedi. Why is this there and is it still needed? I'm happy to create a PR for this.

[davidhalter]

These are the test results for mypy if you try to remove it: https://travis-ci.org/davidhalter/typeshed/jobs/534417186. I'm still in favor of removing it. mypy is by far not the only software using typeshed.

[JelleZijlstra]

mypy is by far not the only software using typeshed.

But it is among the most important users. We're not going to just break one of our most important current users.

There is an existing issue for this in mypy: python/mypy#3171. That issue will have to be addressed first before we can remove it from typeshed.

[AlexWaygood]

Closing as we now have #7580, and I don't think we need both issues to be open when they're about the same thing