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/common.py

@@ -20,6 +20,15 @@ import sys
 import zipfile
 from pathlib import Path
 
+# Every build script imports this module. verify.py, slots.py, validate_pr.py
+# and cross_reference.py started under 3.8 and stopped halfway on a
+# str.removeprefix AttributeError, which named no version.
+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]}"
+    )
+
 
 try:
     import yaml