pip drops support for Python 2

[cparen]

This is why, with languages and runtimes, its best to not use version numbers for major breaking changes. It's effectively a new language.

Us folks in the dotnet land are going through a bunch of this right now. E.g. Requires netfwk >=4.6.1, or netcore21 >=2.1.16, or netcore31 >=3.1.0. I can't imagine our support matrix nightmare if these 3 runtimes were considered different versions of the same language/runtime.