WIP: Support Python 3.

Administrator
2023-03-20T18:19:40Z

*Created by: danielballan*

This is a work in progress. The package builds and installs under py3, but it fails on import like so:

```
In [1]: import av
---------------------------------------------------------------------------
ImportError Traceback (most recent call last)
in ()
----> 1 import av

/Users/danielallan/Documents/Repos/PyAV/av/__init__.py in ()
8
9 # For convenience, IMPORT ALL OF THE THINGS (that are constructable by the user).
---> 10 from .audio.fifo import AudioFifo
11 from .audio.format import AudioFormat
12 from .audio.frame import AudioFrame

/Users/danielallan/container.pxd in init av.audio.fifo (/Users/danielallan/Documents/Repos/PyAV/src/av/audio/fifo.c:2743)()

/Users/danielallan/frame.pxd in init av.container (/Users/danielallan/Documents/Repos/PyAV/src/av/container.c:5922)()

/Users/danielallan/stream.pxd in init av.frame (/Users/danielallan/Documents/Repos/PyAV/src/av/frame.c:2108)()

/Users/danielallan/frame.pxd in init av.stream (/Users/danielallan/Documents/Repos/PyAV/src/av/stream.c:4773)()

/Users/danielallan/plane.pxd in init av.audio.frame (/Users/danielallan/Documents/Repos/PyAV/src/av/audio/frame.c:2564)()

ImportError: dlopen(/Users/danielallan/Documents/Repos/PyAV/av/plane.so, 2): Symbol not found: _PyString_FromStringAndSize
Referenced from: /Users/danielallan/Documents/Repos/PyAV/av/plane.so
Expected in: dynamic lookup
```