Renaming the default branch

[pradyunsg]

There's lots of good reasons to move away from master as the name for the default branch of this project. This GH issue is for figuring out the logistical parts of making this change here.

If you're interested in arguing that we don't make this change, I request that you please file a separate issue.

[pradyunsg]

GitHub is investing in tooling to make the transition easier, and it might likely a good idea to wait for that. From https://github.com/github/renaming/:

If you haven’t renamed your default branch yet, consider waiting until later this year. We’re investing in tools to make renaming the default branch of an existing repository a seamless experience for both maintainers and contributors.

[pradyunsg]

https://github.com/github/renaming/ has happened. I'll be making PRs to prepare for this over the weekend, and making this change this weekend as well.

/cc @pypa/pip-team in case someone has thoughts. :)

[pfmoore]

+1 on doing this. There will be impact (we can minimise but not eliminate it) but in my experience it's manageable.

[pradyunsg]

Done in #9743 and made the switch now. I'll keep an eye out for CI issues, if any new ones arise. :)

[pradyunsg]

Needed to update our bot for this (pypa/browntruck@8adbb68) but the deployment of that change failed. :(

[pradyunsg]

ReadTheDocs' default branch needed to be updated in their admin UI.

[pradyunsg]

Oh hey, this also broke Azure Pipelines, since that was looking for the old branch name. For now, I've just removed it via #9759.

[pradyunsg]

I consider this done now.

[pradyunsg]

ReadTheDocs' default branch needed to be updated in their admin UI.

Upstream issue: readthedocs/readthedocs.org#7907