Drop v2, apply v3 as the only imageio interface.

[evan0greenup]

At this stage, I believe the API of imageio is stable enough.

Everytime when import image, import imageio.v3 as iio is too inconvenient. And accessing its member with different name is not natural. Because, since everyone get used to use it as iio, why does this project named imageio instead of iio.

Frankly speaking, as the functionality and usage of this library. I don't think it will have intensive usage in source code as numpy even for graphic intensive project.

So, I would recommend to move everything in v3 to imageio.__init__.py to it more readable and convenient.