Drop support for Python 2

[pdurbin]

We don't need to go so far as signing https://python3statement.org for this little project but I think we should drop Python 2 support.

https://pythonclock.org is showing around 6 months left:

I'm well aware that Python 2 will continue to be patched by Red Hat and others but it's extra effort to maintain support for Python 2, especially with the "batteries included" approach I've used so far of not requiring third party dependencies (standard library only). Also, better error handling for #12 and #26 for example, would be easier to implement in one version of Python, I think.