Ensure stubtest gets an older version of webcolors when testing jsonschema in CI (#12115)

Author: AlexWaygood

stubs/jsonschema/METADATA.toml

@@ -6,3 +6,5 @@ partial_stub = true
 [tool.stubtest]
 ignore_missing_stub = true
 extras = ["format"]
+# see https://github.com/python/typeshed/issues/12107 for the reason for the pin
+stubtest_requirements = ["webcolors<24.6.0"]