Use a script to generate calendar entries

The number of times I've screwed this up somehow (wrong date, wrong table format, something) is hard to remember, but it's a lot. Using a script for manipulating the calendar takes a lot of the human error out of it.

This script is able to correctly generate release candidate entries based on our scheduled quarterly releases, convert from the rc schedule to the final release schedule, and add additional entries for by release-candidates and stable releases. Comes complete with working type annotations and unit tests.