Add CPython tag denylist to check-releases

Some CPython tags don't build cleanly on riscv64. Rather than retrying
them on every scheduled run, maintain a small in-script DENYLIST of
CPython tags (starting with v3.13.0a2 and v3.13.0b3) that are skipped
after the release-existence check.

Author: claude

.github/scripts/check-releases.py

@@ -38,6 +38,13 @@
 )
 BUILD_WORKFLOW = "build-python.yml"
 
+# CPython tags we know don't build on riscv64. Skipped silently after the
+# release-existence check fails.
+DENYLIST: frozenset[str] = frozenset({
+    "v3.13.0a2",
+    "v3.13.0b3",
+})
+
 # Poll cadence (seconds)
 RUN_DISCOVERY_POLL = 5
 RUN_DISCOVERY_ATTEMPTS = 60  # up to 5 minutes
@@ -233,6 +240,10 @@ def main() -> int:
             log("  already released, skipping")
             continue
 
+        if tag in DENYLIST:
+            log("  in denylist, skipping")
+            continue
+
         ft = minor >= 13
         log(f"  queued for dispatch (freethreaded={str(ft).lower()})")
         to_dispatch.append((tag, ft))