[PieterCK]

Debian 11 (bullseye), the oldest distro we tested Python 3.9 on, has reached end of life, and its packages now return 404 from deb.debian.org, so provisioning in the zulip/ci:bullseye container fails. Python 3.9 itself reached end of life in October 2025.

https://www.debian.org/News/2026/20260831

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[knowprasad]

Took a look through the changes and checked the CI run. Removing the Debian 11 bullseye container completely resolves the 404 package fetch failures we were seeing across backend test runs.

Updating python_requires=">=3.10" and classifiers consistently across all three packages (zulip, zulip_bots, and zulip_botserver), alongside bumping the black target in pyproject.toml, keeps packaging and linting aligned.

All matrix jobs on Debian 12, Ubuntu 22.04/24.04, and Windows passed cleanly.