Remove `av` pin in `transformers` (#13518)
## Summary
This test started failing, and it fails at least back to v0.6, so I
don't think it's on our end. I'm wondering if all the wheels here were
yanked? They're visible in the lockfile, but not on PyPI:
https://pypi.org/project/av/9.2.0/#files. So to get this passing, let's
just unpin it.
Edit: Ahh, ok. It looks like the project ran out of space, so they
removed wheels for all the older versions:
https://github.com/PyAV-Org/PyAV/issues/1879.
