homebrew-python

Install any version of Python under Homebrew.

1. Tap the repo via brew tap GreatFruitOmsk/python
2. Install python you need, e.g. brew install python@3.5.3

Since you cannot have multiple major versions installed simultaneously (e.g. 3.5 and 3.6) brew will fail to link them.
But that's fine as you can either create virtualenv or modify PATH whenever needed.