Resolve issue with type checking in 3rd party referencing.jsonschema.

Since this is a third party library, ignore the error that mypy gives:

... referencing/jsonschema.py:426: error:
Invalid "type: ignore" comment  [syntax]

mypy.ini

@@ -56,3 +56,8 @@ ignore_missing_imports = True
 
 [mypy-update_checker]
 ignore_missing_imports = True
+
+# Added to resolve an issue with mypy in CI:
+# ... python3.8/site-packages/referencing/jsonschema.py:426: error: Invalid "type: ignore" comment  [syntax]
+[mypy-referencing.jsonschema]
+follow_imports = skip