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)