model: Initialize _agents attribute on __new__

This is to avoid the warning:
```
/projectmesa/mesa-examples/examples/boltzmann_wealth_model_experimental/model.py:53: FutureWarning: The Mesa Model class wasn’t initialized. In the future, you need to explicitly initialize the Model by calling super().__init__() on initialization.
  super().__init__(unique_id, model)
```
So that users won't have to call `super().__init__()` in their model
code.

mesa/model.py

@@ -54,6 +54,7 @@ def __new__(cls, *args: Any, **kwargs: Any) -> Any:
             # advance.
             obj._seed = random.random()  # noqa: S311
         obj.random = random.Random(obj._seed)
+        obj._agents: defaultdict[type, dict] = defaultdict(dict)
         return obj
 
     def __init__(self, *args: Any, **kwargs: Any) -> None:
@@ -65,7 +66,6 @@ def __init__(self, *args: Any, **kwargs: Any) -> None:
         self.running = True
         self.schedule = None
         self.current_id = 0
-        self._agents: defaultdict[type, dict] = defaultdict(dict)
 
         # Warning flags for current experimental features. These make sure a warning is only printed once per model.
         self.agentset_experimental_warning_given = False