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 #124
• [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!

[pllim]

This is part of infrastructure, so I am taking liberty to just go ahead with the renaming.