Add stub for buffer, descriptor, plane

[WyattBlue]

def __init__(self, sentinel: object) -> None: ... is wrong. The user can't initialize that object so __init__ shouldn't be in that type stub.

Also, you should always end files with a newline. It's part of our codestyle. GitHub displays a big red sign when a file doesn't.

[laggykiller]

The user can't initialize that object so init shouldn't be in that type stub.

It is true that the user cannot initialize the object, I agree that I should make change on typing of __init__

However, I think we still need to make stub for those classes, as they could be accessed from other objects such as av.audio.plane.

[laggykiller]

What if I change to this: https://github.com/laggykiller/PyAV/commit/88e9f0f82cb8757312570f60005a72bb83133a76