Use Python3 Virtualenv in macos

For different projects may depend on different plugins, and even same plugin with different version. Virtualenv allows us to create different virtualenvs for different projects.

1. Install the virtualenv pip package

   pip3 install virtualenv

2. Create virtualenv

   python3 -m virtualenv /path/to/venv

3. Activate and Use

   source /path/to/venv/bin/activate