Automate version update on main after release

[ClaudiaComito]

After a new release comes out, unless it's a patch release, minor in version.py needs to be bumped up on main, i.e. (from RELEASE.md):

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12. On branch main, we want to modify the version so that minor is increased by 1, and extension is "dev". In this example we want the version on main to be:1.6.0-dev. We need to create a new branch from main:

    git checkout main
    git pull
    git checkout -b workflows/update-version-main
    git branch

    On branch workflows/update-version-main, modify version.py so that minor is increased by 1, and extension is "dev". Commit and push the changes.

13. Create a PR with main as the base branch.

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We forget about this step most times.