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 #67
• [x] 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!

[pllim]

A lot of stuff across the project refer to this repo, so this is one of the repos in the project that needs to change first.

[eteq]

Change done right now @pllim!

[eteq]

One thing to note about this that's especially important for this repo - the github magic recognizes renames such that https://github.com/astropy/astropy-APEs/blob/master/APE8.rst now redirects to https://github.com/astropy/astropy-APEs/blob/main/APE8.rst , so all the links don't have to be updated (although they should be where possible!)

[pllim]

Please also review #67

[eteq]

Oops! Sorry @pllim, I missed #67 and did some of that in main already. But will follow-up in #67