docs: def calculate_tree_differences(self, pytree, doctree):
          """Calculate the differences between the given trees.
  
          :param dict pytree: The dictionary of the parsed Python tree.
          :param dict doctree: The dictionary of the parsed documentation tree.
          :rtype: tuple
          :returns: A two-tuple of sets, where the first is the missing Python files, and the second
              is the missing documentation files.
          """
          pykeys = set(pytree.keys())
          dockeys = set(doctree.keys())
  
          # Calculate the missing documentation files, if any.
          missing_doc_keys = pykeys - dockeys
          missing_docs = {pytree[pyfile] for pyfile in missing_doc_keys}
  
          # Calculate the missing Python files, if any.
          missing_py_keys = dockeys - pykeys
          missing_pys = {docfile for docfile in missing_py_keys}
  
          return missing_pys, missing_docs