Remove Python 2.7 support for h3 4.0

[kylebarron]

I'd like to propose that h3 4.0 deprecate support for Python 2.7. h3 3.x will remain for users who need it, but 2.7 has been EOL for over 2 years by this point. Should we support 3.7+?

I think people were in favor of this on a recent call, but I wanted to start a written discussion as well.

I'd also be in favor of using a tool like pyupgrade to enforce Python 3 syntax throughout the repo, automatically upgrading where possible.

[ajfriend]

I'm cool with dropping 2.7 support in 4.0.