update python 3 mac code example

Example 1: mac upgrade python to 3.8

brew install python3
brew update && brew upgrade python
alias python=/usr/local/bin/python3

Example 2: which python mac

$ brew update && brew upgrade python

Example 3: mac why is python installed in usr and application

$ pyenv global 3.7.3
# and verify it worked 
$ pyenv version
3.7.3 (set by /Users/mbbroberg/.pyenv/version)