Fix CI issues with readthedocs, python 3.7, and broken clip test#1820

reinecke wants to merge 3 commits intoAcademySoftwareFoundation:mainAcademySoftwareFoundation/OpenTimelineIO:mainfrom

reinecke:fix/pin-python-37-old-ubuntureinecke/OpenTimelineIO:fix/pin-python-37-old-ubuntuCopy head branch name to clipboard