This documentation assumes use of Homebrew: https://brew.sh/.
/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
The default installation options are appropriate.
brew install python3
Installing python3 via Homebrew should automatically install pip3. Use that to install virtualenv.
pip3 install virtualenv
Create the virtual environment. From the working directory of our project (e.g., ~/Desktop/invoke-base):
virtualenv -p python3 env36
This will create a directory for the virtual environment (e.g., ~/Desktop/invoke-base/env36/).
Next activate that virtual environment and install our Python dependencies:
source env36/bin/activate
pip3 install -r requirements3.txt