Again bump the referencing version.

Author: Julian

docs/requirements.txt

@@ -82,7 +82,7 @@ pytz==2022.7.1
     # via babel
 pyyaml==6.0
     # via sphinx-autoapi
-referencing==0.20.0
+referencing==0.21.0
     # via
     #   jsonschema
     #   jsonschema-specifications

pyproject.toml

@@ -34,7 +34,7 @@ dependencies = [
   "attrs>=22.2.0",
   "pyrsistent>=0.19.3",
   "jsonschema-specifications>=2023.03.1",
-  "referencing>=0.20.0",
+  "referencing>=0.21.0",
 
   "importlib_metadata;python_version<'3.8'",
   "typing_extensions;python_version<'3.8'",