I have python 3.7 installed but will need to run something with python 3.6. Here are the steps to do so.

brew unlink python
brew install --ignore-dependencies https://raw.githubusercontent.com/Homebrew/homebrew-core/f2a764ef944b1080be64bd88dca9a1d80130c558/Formula/python.rb
brew switch python 3.6.5

To switch it back to python 3.7

brew switch python 3.7.0

Now with two versions installed, we can use virtualenv for projects.

Notice that when you issue virtualenv –python=$(which python3) env, virtualenv may throw an error -bash: /usr/local/bin/virtualenv: /usr/local/opt/python/bin/python3.7: bad interpreter: No such file or directory. That’s because the virtualenv script has a shebang line referencing python 3.7. To fix that error, edit that file /usr/local/bin/virtualenv and change the version to 3.6. Then virtualenv will work fine.

Ref:
https://stackoverflow.com/questions/51726203/installing-python3-6-alongside-python3-7-on-mac
https://stackoverflow.com/questions/45269867/python-virtualenvwrapper-bad-interpreter/48259762#48259762