Use a class creating function, per review.

Also fix doing copy/deepcopy of the objects, and provide more information when the args are wrong (also per review).

scenario/context.py

@@ -5,7 +5,7 @@
 import tempfile
 from contextlib import contextmanager
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Dict, Final, List, Optional, Type, Union, cast
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Type, Union, cast
 
 from ops import CharmBase, EventBase
 
@@ -19,7 +19,7 @@
     Storage,
     _CharmSpec,
     _Event,
-    _MaxPositionalArgs,
+    _max_posargs,
 )
 
 if TYPE_CHECKING:  # pragma: no cover
@@ -36,7 +36,7 @@
 
 
 @dataclasses.dataclass(frozen=True)
-class ActionOutput(_MaxPositionalArgs):
+class ActionOutput(_max_posargs(0)):
     """Wraps the results of running an action event with `run_action`."""
 
     state: "State"
@@ -50,8 +50,6 @@ class ActionOutput(_MaxPositionalArgs):
     failure: Optional[str] = None
     """If the action is not a success: the message the charm set when failing the action."""
 
-    _max_positional_args: Final = 0
-
     @property
     def success(self) -> bool:
         """Return whether this action was a success."""