build: switch to starflow

[lengau]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run tox?

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[bepri]

Thank you! Always nice to have more stuff on Starflow/Starbase.

Aside from the comments I'm leaving below, there's also a general one: "tox" (as a string) still appears in three files: .gitignore, HACKING.rst, and pyproject.toml. Could you remove those references to tox, or replace them with their modern equivalents?

[lengau]

@sergiusens we need your super admin powers on this please. The 3 pending checks need to be removed as mandatory with the following added:

• lint / files
• tests / Fast tests (noble, amd64)
• tests / Slow tests (noble, amd64, 3.12)

Once that's done we should be clear to rebase & merge