[META] Move from `master` branch to `main`

[Secrus]

We should follow the times and move from using the master branch to using main. The old master branch should stay around for some time after the transition to avoid potentially breaking setups that are based on it.

[vivodi]

There are so many issues waiting to be addressed, yet you're spending time on this? Python 3.13 still doesn't have a wheel available, and users can only install from source. By the way, when was the last release?