Prevent seed and random from being shared between instances

This commit aims to change two related issues:

- New instances of a model should not be able to overwrite the value of the
seed of previous (and possibly running) models.
- The `random` attribute should be instance-level, not class-level.

Prior to this commit, both `_seed` and the `random` were class attributes that
were set/changed every time a new instance was created.

This fixes #1438

mesa/model.py

@@ -21,9 +21,10 @@ class Model:
 
     def __new__(cls, *args: Any, **kwargs: Any) -> Any:
         """Create a new model object and instantiate its RNG automatically."""
-        cls._seed = kwargs.get("seed", None)
-        cls.random = random.Random(cls._seed)
-        return object.__new__(cls)
+        obj = object.__new__(cls)
+        obj._seed = kwargs.get("seed", None)
+        obj.random = random.Random(obj._seed)
+        return obj
 
     def __init__(self, *args: Any, **kwargs: Any) -> None:
         """Create a new model. Overload this method with the actual code to

tests/test_model.py

@@ -28,6 +28,9 @@ def step(self):
 def test_seed(seed=23):
     model = Model(seed=seed)
     assert model._seed == seed
+    model2 = Model(seed=seed + 1)
+    assert model2._seed == seed + 1
+    assert model._seed == seed
 
 
 def test_reset_randomizer(newseed=42):