Upcoming dependency resolver in pip

The new dependency resolver [to be switched on by default in pip 20.3 (October 2020)](https://pip.pypa.io/en/latest/user_guide/#changes-to-the-pip-dependency-resolver-in-20-2-2020) might affect the material taught in this lesson. I'm opening this issue as a way to flag up the possibility, and perhaps as a place to discuss the implications for this lesson of the impending change to Pip's default behaviour.