Changes in how homebrew installs Python on Mac OSX

Homebrew no longer sets pip.

See 5 executable names in https://docs.brew.sh/Homebrew-and-Python.html

    python points to the macOS system Python (with no manual PATH modification)
    python2 points to Homebrew’s Python 2.7.x (if installed)
    python3 points to Homebrew’s Python 3.x (if installed)
    pip2 points to Homebrew’s Python 2.7.x’s pip (if installed)
    pip3 points to Homebrew’s Python 3.x’s pip (if installed)