Pyenv is great. On our Macs we've had zero issues installing older, specific versions of Python, every time. Highly recommended. (Getting it working properly with zsh was a bit frustrating, but that's my own fault.)
$ pyenv virtualenv 3.6 some-name
$ echo some-name > /path/to/project/.python-version
$ cd /path/to/project
$ pip install -r requirements.txt