MNT: Rename default branch from master to main

[pllim]

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:

• [x] Rename branch from master to main, preferably by 2021-03-22.
• [x] Update documentation, workflows, etc., accordingly. -- See #77
• [ ] Communicate this change to your users and stakeholders.

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!

[astrojuanlu]

I think this is done! This repository doesn't have a lot of stakeholders :) If problems arise, I'll fix them. Thanks @pllim!