You should give MacPorts a try... MacPorts allows you to install multiple Python versions at the same time and I personally have several venvs with Python 2.7,

You should give MacPorts a try... MacPorts allows you to install multiple Python versions at the same time and I personally have several venvs with Python 2.7, 3.6, 3.7, 3.8, 3.9... all running without a glitch.