This is essentially a re-write of the changelog code, including:

1. Replacing ad-hoc code to access the Github REST API with the pyGithub library
2. Searching pull requests has been dropped, as not all tickets have been closed against pull requests.
3. Implementing an alternative algorithm to determine tickets closed for a given tag. The new algorithm now searches up the master branch for ticket branch merges
4. Weekly tags are now the only tags used.
5. As weekly tags are used, the tag issue prior to w.2020.20 where only pfs_pipe2d was tagged (as part of a release), the rather flaky mechanism of determining tickets-to-tag assignment based on datestamps is removed.

I think we need to consider tags in addition to the weeklies, as every now and then we use them (e.g., I made two mid-week tags for the commissioning run).

We discussed timing, as the first run (on only two repos) took a few minutes, but then subsequent runs were faster (just over a minute). Profiling the subsequent runs shows the time is dominated by communication. The Jira data is cached next to the code, but I think putting it in ~/.pfs/ would make it more useful. We could probably greatly speed up the code by caching the GitHub data, but we only run this code once a week, with an automated system, at a time when the machine is quiet, so I don't think that's a high priority, and maybe not even worth worrying about.

Since this is a re-write, this might be a good opportunity to get a feel for including python typing, and running the code through black and isort.

Merged to master. Will address support for tag types other than weeklies in a separate, future ticket.