fix: load every script under the 3.9 floor

A module-level alias written with a PEP 604 union compiled under 3.9 and
raised TypeError when check_release_assets.py, and check_release.py
through it, were loaded. The floor test now imports every module under
scripts/ with a real 3.9 interpreter, which CI gives pyyaml and
jsonschema.

.github/workflows/validate.yml

@@ -195,8 +195,10 @@ jobs:
 
       # install.py stays on Python 3.8: the suite runs it end to end under a
       # real 3.8 interpreter, which import-only and grammar checks miss. 3.9
-      # is the floor pipeline.py states for the build scripts, compiled under
-      # a real 3.9. The last version listed is the default python.
+      # is the floor pipeline.py states for the build scripts: a real 3.9
+      # imports every module under scripts/, so it needs their dependencies.
+      # jsonschema 4.26 requires 3.10, 4.25.1 is the last release for 3.9.
+      # The last version listed is the default python.
       - uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97  # v7
         with:
           python-version: |
@@ -204,6 +206,9 @@ jobs:
             3.9
             3.12
 
+      - name: Install the build scripts' dependencies for Python 3.9
+        run: python3.9 -m pip install pyyaml "jsonschema[format-nongpl]==4.25.1"
+
       # markdown: the site validation test renders a wiki line to prove the
       # heading guard's premise, and skipped on every run without it. The docs
       # group builds the site as deploy-site.yml does, with the same bounds.