Rename primary branch from master to main

[sliverc]

Description of the Change

This is now the standard naming of git repositories and Django has also moved to main naming.

What needs to be done besides this PR:

• Update primary branch in GitHub settings
• Update to main branch in ReadTheDocs
• Check CodeCov integration

Add more TODOs in case I am missing anything.

For our local setup each of us will need to run the following:

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

[sliverc]

@n2ygk If you agree with this change simply approve. I will then merge and do all the settings in one go.

[n2ygk]

agree

[sliverc]

Renamed. Pyup.io should get the primary branch name automatically from GitHub but need to keep an eye on this. A new PyUp PR should arrive during the next week.