fix: lower the pipeline floor to Python 3.9

The 3.12 guard rested on generate_readme f-strings that have since been
rewritten. Every script compiles under CPython 3.9.25, 3.10 and 3.11, and
only generate_pack's parenthesized context managers fail under 3.8. Under
3.9 the offline pipeline (--skip-packs) completed with no change to the
committed artifacts, the 15 manifests matched 3.12 byte for byte, and the
RomM pack was identical. CI now compiles every script under a real 3.9.

wiki/testing-guide.md

@@ -24,7 +24,7 @@ python -m unittest tests.test_install tests.test_install_cores -v
 
 The table below says which module covers what.
 
-Running the build needs Python 3.12 and `pyyaml` alone, and so does most of the suite. The
+Running the build needs Python 3.9 or newer and `pyyaml` alone. CI runs the suite on 3.12. The
 schema contract tests also need `jsonschema[format-nongpl]`, and one site test
 renders a wiki line with `markdown`. The CI installs both: without them those
 tests are reported as skipped, so install them before a pull request