Added a to_str function for all classes. __str__ function uses it. highscores_data uses the to_str function for saving.

data/category.py

@@ -30,7 +30,7 @@ class Category:
         Returns:
             str: Category class as a string
         """
-        return json.dumps(self.to_dict(), indent=4)
+        return self.to_str()
 
     @classmethod
     def from_dict(cls, name: str, data: dict):
@@ -158,3 +158,12 @@ class Category:
             data["scores"][score.user] = score.value
 
         return data
+
+    def to_str(self) -> str:
+        """
+        Returns Category object as a string.
+
+        Returns:
+            str: string of json representation of Category
+        """
+        return json.dumps(self.to_dict(), indent=4)