Create a feature-v3.0 branch

[sveinse]

With reference to the discussion #569 (comment) about wanting to release v2.4.0 before starting with a v3.0:

Any objections to creating a feature-v3.0 branch / development / next (name can be discussed) from master? This way the "v3.0" work can start now and not be obstructed by the finalizing of a v2.4.0 release. It's a temporary branch. Once v2.4.0 is out, we merge the feature-v3.0 branch into main and close it. I can take responsibility to keep this branch in sync with master and ensure that the commits we add to v2.4.0 are applied to the feature-v3.0 branch.

[acolomb]

I just created a branch v0.3 for this purpose and enabled the same protection settings as on master. However, it is a little more complicated, we would need to adjust the workflow configuration to actually make the checks run in GitHub Actions. And apparently the Codecov app doesn't trigger for a PR against that branch either, see #602.

So overall I think the gains do not outweigh the effort. The v2.4.0 release should go out really soon, mainly blocked by the cosmetics PR #596. I think we're fine targeting v3.0 changes against master and just keeping the PRs open until after the v2.4.0 release?