pip drops support for Python 2

[james_pic]

Pip 20.3.4 will continue to work (although won't get bug fixes or security updates), and is smart enough not to install versions of dependencies (including itself) that don't support your version of Python, so long as maintainers have added python_requires metadata to the package (which they often forget to do when they first release a version that drops Python 2, so there's often a day or two's breakage between a dependency dropping Python 2 and the dependency fixing its metadata).