Install issue w/python3.8.2, OSX 10.15.7, suggest adding subversion # for python 3

[jasonhemann]

I found I could not build Twisted, thus rmview, w/Python 3.8.2. It seemed likely related to this: https://stackoverflow.com/questions/64111438/how-to-fix-error-pip-install-twisted-on-mac

I can reproduce the logs if you like. I suggest adding a version or subversion number to the python3 requirement in the readme. Please let me know if either the logs or this suggestion are useful, if so, I'm happy to follow up.