Add typing stubs for pyobjc-core

[ronaldoussoren]

The first step for adding typing support to all of PyObjC is to add typing support for PyObjC-core:

• [ ] Add a .pyi file for objc._objc
• [ ] Add type annotations to files in Lib/.
• [ ] Consider adding type annotations to tests
• [ ] Include mypy run in test regime

Some challenges in all of this:

• Every ObjC class proxy has a meta class as well, which is something not supported by most typing tools. For the most part this can be worked around by using @classmethod in type stubs. The challenge there is that there are some ObjC methods that are valid both as class and instance methods, such as +[NSObject description] and -[NSObject description].
• Formal protocols (@protocol in Objective-C) need some kind of named representation for typing, in particular to deal with methods that return or accept a type conforming to a protocol, e.g. -(id <NSCopying> returnCopyableObject;. This is less critical for pyobjc-core. #419

[ronaldoussoren]

Another challenge is "objc.lookUpClass", its result is a particular type. For now I have it return objc_object, but requires more tweaks in pyobjc-core to work properly...

I'm not sure how to handle this without writing a mypy plugin, or keeping the current mess of casting.