Make AudioFrame / VideoFrame more gc-friendly

[jlaine]

@Mofef pointed out in #317 (comment) that our VideoFrame contain reference cycles which prevent the garbage collector from reclaiming the frames as soon as they are unused. Let's try and improve this!

Here is a trivial example which currently has a fairly large memory (but stable) memory consumption:

import av
  
with av.open('test.mp4') as container:
    for frame in container.decode(video=0):
        print(frame.pts)

On a 1080p video, RSS stabilizes around 480MB for me.

[jlaine]

Applying #516, RSS drops to 85MB.

[jlaine]

Fixed in 75036f6