Rename branch to main #15

I'd like to propose we rename the master branch to main. The word 'master,' is charged, and its use should be discouraged. This was done for the CPython repository back in 2021 after both the community, and the Steering Council agreed this was the way to go.

GitHub has thankfully added quite considerable support for this (see the documentation), and will provide redirects/support for the old name. As such, nothing should break. (Well -- hopefully we won't break GitHub this time! ;-)