Consider renaming `master` branch to `main`

[bgilbert]

The community as a whole is moving away from master, and main seems to be a common choice. Now would be a good time to do it, before the library gets more adoption.

GitHub allows fairly transparent branch renaming from this page, though cloned repos would need to be updated manually.

[hackermd]

Fine with me. We will need to update a few things (Read the Docs, etc.) but that should be relative straightforward. Shall we first finalize the currently open pull requests and then rename?

[bgilbert]

There's actually no requirement that we land the outstanding pull requests first (GitHub will automatically update the target branch) so this could be done at any time.