Thanks for testing! And just to rephrase for others, this is a one-time (per environment/per virtualenv) thing - you want to upgrade pip to a version new enough

Thanks for testing! And just to rephrase for others, this is a one-time (per environment/per virtualenv) thing - you want to upgrade pip to a version new enough to know not to upgrade too much, and then you're fine. Once you run the command I gave, a later "pip install -U pip" will keep you on 20.x and not upgrade to 21.