STY: Avoid shadowing outer variables in PET test private class

Avoid shadowing outer variables in PET test private class: add a leading
underscore to the dummy private class attribute.

Author: jhlegarreta

test/test_pet_workflow.py

@@ -47,13 +47,13 @@ def test_pet_motion_estimator_run(monkeypatch):
     ds = _pet_dataset(3)
 
     class DummyModel:
-        def __init__(self, dataset):
-            self.dataset = dataset
+        def __init__(self, _dataset):
+            self._dataset = _dataset
 
         def fit_predict(self, index):
             if index is None:
                 return None
-            return np.zeros(self.dataset.shape3d, dtype=np.float32)
+            return np.zeros(self._dataset.shape3d, dtype=np.float32)
 
     monkeypatch.setattr("nifreeze.estimator.PETModel", DummyModel)