Rename master branch main

[jarrodmillman]

Close #549.

Here is what I think needs to be done:

1. Rename default branch main on GitHub:
   • https://github.com/networkx/networkx/settings/branches
2. Merge this PR
3. I don't think you need to do anything for codecov.io.
4. I don't use RTD, but I think you will need to follow these steps:
   • Detect when the default branch is renamed (e.g. from master to main) readthedocs/readthedocs.org#7907 (comment)
5. Update your locally cloned repository to have a main branch as follows:

git branch -m master main
git fetch <YOUR_UPSTREAM_REMOTE>
git branch -u <YOUR_UPSTREAM_REMOTE>/main main

(where YOUR_UPSTREAM_REMOTE is typically called `upstream` or `origin`)

The whole process should only take a few minutes. You could send an email to developers telling them about step 5, if you want.

[choldgraf]

+1 from me, I think we are overdue for this.

I'm happy to do the manual actions that @jarrodmillman notes above if there are no objections

[jarrodmillman]

@choldgraf Any interest in making this change today? It shouldn't take long and it may be good to do it before the 0.9rc1.

[choldgraf]

I'm +1 on doing this today, as long as it won't create big headaches for @12rambau in #659 since he needs to resolve conflicts. If we don't think that'll be a big deal, then +1 from me (I just don't have the experience to know)

[12rambau]

Normally I solved everything in my PR if we can merge it before this one that would be perfect. Also @jarrodmillman could you copy/paste the instructions to change from master to main in an issue so that it can be pinned on top of the issue list it will be perfect (I'm positive I'm going to forget and I think I wont be the only one)

[choldgraf]

Great idea to pin those instructions in an issue. We can plan to close it in, say, 3 months.

[jarrodmillman]

I added a draft of the pinned issue:
#677

[choldgraf]

Thanks so much for your help @jarrodmillman 🙏

[jarrodmillman]

@choldgraf I think we are set to make the change. I don't have access to the repository settings, so you will need to do that. In my experience the process is painless and we have almost no remaining PRs (but they will be updated automatically). The only issue I've ever seen is that PR whose branch has been deleted are closed, but none of our existing PRs have that issue.

You should be able to follow the steps I list on my first comment on this issue.

[choldgraf]

OK - I have:

• Renamed the default branch for this repo in github
• Did a quick look at the code for master to make sure we didn't miss something. It looked good, but I noticed we still used the Sphinx master_doc variable, which had since been renamed to root_doc. So I took the liberty of using that instead.
• Will merge this if tests are happy
• Will see if ReadTheDocs breaks, and if it does then I'll manually name the branch to main in RTD