chore(internal): update test syntax (#1798)

Author: stainless-app[bot]

tests/test_models.py

@@ -245,7 +245,7 @@ class Model(BaseModel):
     assert m.foo is True
 
     m = Model.construct(foo="CARD_HOLDER")
-    assert m.foo is "CARD_HOLDER"
+    assert m.foo == "CARD_HOLDER"
 
     m = Model.construct(foo={"bar": False})
     assert isinstance(m.foo, Submodel1)