My development systems have been using Python 3.9 for a while, so I'm quite sure that all Heptapod tests will pass easily, but these don't cover much of Mercurial itself. Hence it is reasonable to start with Mercurial 5.9.
- switch all Heptapod CI images to Python 3.9 (should be as simple as using Debian 11)
- change the bundled Omnibus version to Python 3.9 (take the occasion to check where upstream currently sits).
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