change the default python version mac

#install or update brew
$ brew install pyenv
#install
$ pyenv install 3.7.3
#make default
$ pyenv global 3.7.3
# and verify
$ pyenv version
3.7.3 (set by /Users/mbbroberg/.pyenv/version)
$ echo -e 'if command -v pyenv 1>/dev/null 2>&1; then\n  eval "$(pyenv init -)"\nfi' >> ~/.bash_profile
#reset the current shell
$ exec $0 
$ which python
#/Users/mbbroberg/.pyenv/shims/python
$ python -V
$ pip -V
#pip 19.0.3 from /Users/mbbroberg/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip (python 3.7)