Fix mutable-default bug in codegen.config with Python 3.11 and newer

Since Python 3.11, dataclasses check if default field values are hashable, and if they are not, reject them as mutable. For this reason, the config API using the Category descriptor was broken with Python 3.11, which we didn't see since the CI did not test against 3.11.

This MR

• Fixes this behavior by circumventing the dataclass mechanism
• Adds a CI testsuite task using Python 3.13 to detect such errors in the future

src/pystencils/codegen/config.py

@@ -196,12 +196,16 @@ class Category(Generic[Category_T]):
 
     def __get__(self, obj: ConfigBase, objtype: type[ConfigBase] | None = None) -> Category_T:
         if obj is None:
-            return self._default
+            return None
 
-        return cast(Category_T, getattr(obj, self._lookup, None))
+        cat = getattr(obj, self._lookup, None)
+        if cat is None:
+            cat = self._default.copy()
+            setattr(obj, self._lookup, cat)
+        return cast(Category_T, cat)
 
-    def __set__(self, obj: ConfigBase, cat: Category_T):
-        setattr(obj, self._lookup, cat.copy())
+    def __set__(self, obj: ConfigBase, cat: Category_T | None):
+        setattr(obj, self._lookup, cat.copy() if cat is not None else None)
 
 
 class _AUTO_TYPE: ...  # noqa: E701