Rename from master to main

[consideRatio]

The master branch can be easily renamed, and many (all?) references are redirected by GitHub from the old name to the new when renaming a branch. For example, if someone installs this repo from the master branch via https://github.com/jupyterhub/batchspawner/archive/master.zip, it will still work after the rename.

• A search in this repo didnd't reveal references to master
• Verified that nothing was missed in the checklist here: Rename GitHub branches from master to main team-compass#412

For your local git repositories, you can do these steps are provided by github's web-based hint following the rename.

# rename your local master to main
git branch -m master main

# get the latest stuff from this repo
git fetch origin

# let the main branch track origin/main
git branch -u origin/main main

# I can't explain this...
git remote set-head origin -a

[consideRatio]

I went for the rename, I hope this was considered okay please let me know if you think I moved to fast!