Drop ffmpeg-python dependency and call ffmpeg directly. (#1242)

* Drop ffmpeg-python dependency and call ffmpeg directly.

The last ffmpeg-python module release was in 2019[1], upstream seem to be
unavailable[2] and the project development seem to have stagnated[3].  As
the features it provide is trivial to replace using the Python native
subprocess module, drop the dependency.

 [1] <URL: https://github.com/kkroening/ffmpeg-python/tags >
 [2] <URL: https://github.com/kkroening/ffmpeg-python/issues/760 >
 [3] <URL: https://openhub.net/p/ffmpeg-python >

* Rewrote to use subprocess.run() instead of subprocess.Popen().

* formatting changes

* formatting update

* isort fix

* Error checking

* isort 🤦🏻

* flake8 fix

* minor spelling changes

---------

Co-authored-by: Jong Wook Kim <jongwook@openai.com>

requirements.txt

@@ -4,4 +4,3 @@ torch
 tqdm
 more-itertools
 tiktoken==0.3.3
-ffmpeg-python==0.2.0