ln -sf /usr/local/bin/python3 /usr/local/bin/python
ln -s -f /usr/local/bin/python3.7 /usr/local/bin/python
# it is said not to do but I used it anyway, because of the conveniency
$ echo "alias python=/usr/local/bin/python3.7" >> ~/.zshrc
unlink /usr/local/bin/python
ln -s /usr/local/bin/python3.3 /usr/local/bin/python
ln -s -f /usr/local/bin/python3.7 /usr/local/bin/python
[ -e ~/.profile ] && . ~/.profile