An alternative to MacPorts is Homebrew, and personally I prefer to use it in an isolated directory so that I can easily remove it without poluting the system:
cd $HOME
git clone https://github.com/mxcl/homebrew.git
echo "export PATH=$HOME/homebrew/bin:$PATH" >> $HOME/.bashrc
source $HOME/.bashrc
brew update
brew upgrade
brew install python@3.8
brew cleanup