MNT: Rename default branch from master to main

Please rename your default branch from `master` to `main` as part of astropy/astropy-project#151 , preferably by 2021-03-22. Also a friendly reminder to check documentation, workflows, etc., and update them accordingly. Please don't forget to communicate this change to your users and stakeholders. To summarize:

- [ ] Rename branch from `master` to `main`, preferably by 2021-03-22.
- [x] Update documentation, workflows, etc., accordingly. -- N/A
- [x] Communicate this change to your users and stakeholders. -- N/A

Once this is taken care of, you may close this issue.

*This is an automated issue. If this is opened in error, please let @pllim know!*

Start in the GitHub repository settings and review the default branch configuration for master. Check the repository for references to master, then verify that the default branch is main and that no required documentation or workflow references remain outdated.

Written by the indexing model from the issue text.