Renamed default branch to main

[blink1073]

Anyone who has the repository already cloned will likely need to update it. These are the commands Github suggests:

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

[davidbrochart]

Should "master" be replaced with "main"?

jupyter_client/.github/workflows/check-release.yml

Line 4 in 52828f0

branches: ["master"]

[blink1073]

Good catch, yes

[vidartf]

Probably ok to close now?