sudo mkdir /usr/local/Frameworkssudo chown $(whoami):admin /usr/local/Frameworks brew link python3
sudo mkdir /usr/local/Frameworkssudo chown $USER /usr/local/Frameworks
And then try re-installing python. This worked absolutely fine for me.
Run this, and follow its suggestions:
brew doctor
In my case, it wanted me to run:
sudo mkdir -p /usr/local/sbin /usr/local/Frameworkssudo chown -R $(whoami) /usr/local/sbin /usr/local/Frameworks