I needed to upgrade from python 3.4 to 3.6 to be able to use the SIP library required by PyQt5. Using brew virtualenv-3.x failed and pyenv has been deprecated as of python 3.6.
brew install python3 brew link --overwrite python3 deactivate python3.6 -m venv ./pyenv source ./pyenv/bin/activate
The defaults of the venv command, pip installed, and no system packages, are exactly what I required. For more information refer to pythons venv command.
See also Migrating PyPI packages to a newer python minor version for a quick method to populate your new environment with the packages from a previous environment.