Bump version to 2020.1

[pganssle]

I was mostly holding off on this release because I wanted to resolve #25 and I wanted to get some feedback before we finalize everything (seems prudent to keep tzdata in beta at least for some of the zoneinfo beta period...), but because of this issue, people doing naive pip install tzdata installations will find themselves with an old, incompatible package. I think getting pip install tzdata to work right is better than whatever problems I can imagine from having to make a 2020.1.1 release or something.