Historically, upgrading an OS is always a total pain
In particular, as of June 2018, Python 3.7 has been released, and that's screwing up many of the package managers where installing Python 3.5 of 3.6 has been the default (and intended) behavior.
As a result, here's how to install the versions you want
Installing python 3.6.5
brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/f2a764ef944b1080be64bd88dca9a1d80130c558/Formula/python.rb
Switching from 3.7 to 3.6 and vice versa
brew switch python 3.7.0 brew switch python 3.6.5_1