python@3.8 is keg-only, which means it was not symlinked into /usr/local,
because this is an alternate version of another formula.

If you need to have python@3.8 first in your PATH run:
  echo 'export PATH="/usr/local/opt/python@3.8/bin:$PATH"' >> ~/.zshrc

For compilers to find python@3.8 you may need to set:
  export LDFLAGS="-L/usr/local/opt/python@3.8/lib"

For pkg-config to find python@3.8 you may need to set:
  export PKG_CONFIG_PATH="/usr/local/opt/python@3.8/lib/pkgconfig"

➜  ~ python --version
zsh: command not found: python3.8
➜  ~ vim .zshrc
➜  ~ source .zshrc
➜  ~ echo 'export PATH="/usr/local/opt/python@3.8/bin:$PATH"' >> ~/.zshrc
➜  ~ source .zshrc