fix(robot): use explicit class reference for super() in VariablesDoc to allow slots

Author: d-biehl

packages/robot/src/robotcode/robot/diagnostics/library_doc.py

@@ -1260,7 +1260,7 @@ def to_markdown(
         only_doc: bool = True,
         header_level: int = 2,
     ) -> str:
-        result = super().to_markdown(add_signature, only_doc, header_level)
+        result = super(VariablesDoc, self).to_markdown(add_signature, only_doc, header_level)
 
         if self.variables:
             result += "\n---\n\n"