fix: refuse Python 3.8 in every build script

Only generate_pack.py and pipeline.py checked the interpreter. verify.py,
slots.py, validate_pr.py, cross_reference.py and the probe tools started
under 3.8 and stopped halfway on a str.removeprefix AttributeError. The
check now lives in common.py, which every build script imports, and in
core_probe.py and region_check.py, which do not. generate_site.py imports
common before siterender, which raised a TypeError at load under 3.8.

scripts/core_probe.py

@@ -40,6 +40,14 @@ from collections.abc import Iterable, Iterator, Sequence
 from dataclasses import dataclass, field
 from pathlib import Path
 
+# The probe tools do not import common.py, which states the 3.9 floor for the
+# other scripts. Under 3.8 they stopped on a str.removesuffix AttributeError.
+if sys.version_info < (3, 9):
+    sys.exit(
+        f"{Path(sys.argv[0]).name} needs Python 3.9 or newer, "
+        f"this is {sys.version.split()[0]}"
+    )
+
 ROOT = Path(__file__).resolve().parent.parent
 MIN_BENCH_REPETITIONS = 5
 DEFAULT_WORK_DIR = ROOT / "tmp" / "core_probe"