Please migrate away from unmaintained dep ffmpeg-python

As written here:

https://github.com/googlefonts/gftools/blob/b58af4623cf44e3aec3c0806665782c5b3ea417f/pyproject.toml#L73

Currently gftools depends on https://pypi.org/project/ffmpeg-python/ , which is an unmaintained ffmpeg python wrapper last updated in 2019 with [security vulnerabilities](https://github.com/kkroening/ffmpeg-python/issues/877).

As a result, I am suggesting the migration away from this dependency library. There are a bunch of other ffmepg python wrappers available over the internet.