Fix two failing Python Tests on Ubuntu 20.04

[whoenig]

• Update test_videoOutput to use the correct Python version
• Add missing information to docs

Fixes issue #496

[whoenig]

@jpreiss Any idea how to fix those failing tests? It looks to me like ffmpeg-python simply doesn't work with Python2 anymore.

[jpreiss]

It seems like Python 2 CI is failing on the installation of python-ffmpeg from Pip.

I think it's reasonable to leave video out of the standard Crazyswarm dependency list. The pytest.mark.skip decorator for the video test currently checks "am I running on CI?", but we can replace it with one that checks "are ffmpeg and ffmpeg-python installed?"

This is what I did for the ROS import test - see 402f099.

[whoenig]

I only added it to CI because it's missing in the documentation and the documentation is generated from CI ... Should we leave it out from the docs or have a separate manual line there?

[jpreiss]

Yes, I was thinking of a separate line in the manual presented as an optional step. We never get issues/discussions about video output so I feel that most users do not use it.

[jpreiss]

ffmpeg-python accesses ffmpeg through the shell, so we might want to tell the user to sudo apt install -y ffmpeg as well.