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.

scripts/generate_readme.py

@@ -582,7 +582,7 @@ def generate_readme(db: dict, platforms_dir: str) -> str:
             "python scripts/verify.py --platform retroarch --verbose  # emulator ground truth",
             "```",
             "",
-            "Requires Python 3.12 and `pyyaml`. The one-line installer alone runs on Python 3.8.",
+            "Requires Python 3.9 or newer and `pyyaml`. The one-line installer alone runs on Python 3.8.",
             "",
             "## Documentation site",
             "",

scripts/pipeline.py

@@ -51,11 +51,13 @@ import sys
 import time
 from pathlib import Path
 
-# The build scripts use syntax CPython 3.12 introduced (generate_readme's
-# f-strings, generate_pack's parenthesized context managers). On an older
-# interpreter the run got through the packs and failed hours in, at step 7.
-if sys.version_info < (3, 12):
-    sys.exit(f"pipeline.py needs Python 3.12 or newer, this is {sys.version.split()[0]}")
+# generate_pack's parenthesized context managers do not compile before
+# CPython 3.9, which builds the same database, manifests and packs as 3.12.
+# On an older interpreter the run got through the first steps and failed in
+# the packs, so it stops before step 0. A test compiles every script under
+# this floor.
+if sys.version_info < (3, 9):
+    sys.exit(f"pipeline.py needs Python 3.9 or newer, this is {sys.version.split()[0]}")
 
 sys.path.insert(0, str(Path(__file__).parent))
 from common import (