simplify package installation by removing ffmpeg cli install

[fcakyon]

edit: you no longer have to accept this PR, just created a new package including this feature: https://github.com/fcakyon/pywhisper

This PR reaplaces ffmpeg cli tool + ffmpeg-python package with moviepy package without any difference in audio waveform values.

This PR removes the external cli tool dependency and simplifies installation since pip install would be enough.

I have tested with tests/jfk.flac file and load_audio() output is exactly same as before:

dtype: dtype('float32')
max: 0.78271484
min: -0.7235718
shape: (176000,)
size: 176000

Also tested with a 1h43min video, works without any issues.

moviepy: https://github.com/Zulko/moviepy

[jongwook]

Thanks for the suggestion (and for the other PRs!) moviepy also uses ffmpeg through the command line, indirectly via imageio. Given that we won't need all the image/movie-specific features, ffmpeg-python seems to be a leaner choice.