Update Copyright/License on ffmpeg_burnins.py

[jminor]

The file ffmpeg_burnins.py has an MIT license which differs from the rest of the OTIO source code. It should either be moved into a separate folder to clarify that it is an embedded dependency, so it should be re-licensed to conform with the rest of the project. (See also #632)

https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/master/contrib/opentimelineio_contrib/adapters/ffmpeg_burnins.py

[repsac]

Should be done now

[jminor]

Thanks @repsac !
Addressed by #679