mac change to python 3 code example

Example 1: how to set default python version in macos

ln -s -f /usr/local/bin/python3.7 /usr/local/bin/python

Example 2: 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)