Use Ouranos GitHub bot for bump version, add pygments pin

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Replaces the method using BUMP_VERSION_TOKEN to bump version with the Ouranos bot approach

Does this PR introduce a breaking change?

No.