If you use Homebrew to install Python and Python3, make sure to install python3 first. Otherwise, when trying to upgrade pip3 withpip3 install --upgrade pip it will clobber default command pip and link it to the python3 packages, i.e., pip3.
To avoid a lot of wasted time and relinking I do this:
# First install Python3
$ brew install python3
$ brew linkapps python3
$ pip3 install --upgrade pip setuptools
# Then install Python 2