refactor: use identity check for comparison to a singleton

Comparisons to the singleton objects, like `True`, `False`, and `None`, should be done with identity, not equality. Use `is` or `is not`.

tests/test_hsf.py

@@ -170,7 +170,7 @@ def test_roiloc(models_path):
     mri, mask = hsf.roiloc_wrapper.get_mri(mris[0],
                                            mask_pattern="no_mask.nii.gz")
     assert isinstance(mri, ants.ANTsImage)
-    assert mask == None
+    assert mask is None
 
     _, right, left = hsf.roiloc_wrapper.get_hippocampi(mri, {
         "contrast": "t2",