Rename default branch from "master" to "main"

[hugovk]

To match most of the other https://github.com/python repos, let's use main as the default branch.

Please could @python/organization-owners rename it?

Docs here:

• https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch

[ewdurbin]

Complete