Create a release branch in the scikit-learn-release repo matching the name of the scikit-learn repo #12

I guess we should follow numpy https://github.com/numpy/numpy-release#branches-and-tags.

For NumPy releases, the branch naming should match those of the main numpy/numpy repository, e.g., maintenance/2.3.x for the 2.3.x releases.

See for example https://github.com/numpy/numpy-release/tree/maintenance/2.4.x

We didn't do this for the 1.9.1 because we didn't know ...