Specify skip for separate referencing package (#2049)

Fixes the mypy error that is causing all pipelines to fail. Underlying
reason seems to be that the `jsonschema` library was updated and
factored out some code into a separate package. So updated it to also
reference that file in our mypy overrides.

Might be fixed for newer versions by this recent commit
python-jsonschema/referencing@f830e4c

Author: Ruwann

pyproject.toml

@@ -109,3 +109,7 @@ exclude_lines = [
 [[tool.mypy.overrides]]
 module = "referencing.jsonschema.*"
 follow_imports = "skip"
+
+[[tool.mypy.overrides]]
+module = "referencing._core.*"
+follow_imports = "skip"