FIX: Avoid attempted coercion of str to dtype in comparison

Author: effigies

nibabel/nifti1.py

@@ -2124,13 +2124,15 @@ def set_data_dtype(self, datatype):
         >>> img.get_data_dtype() == np.dtype('int64')
         True
         """
-        # Static aliases
-        if datatype == 'mask':
-            datatype = 'u1'
-        # Dynamic aliases
-        elif datatype in ('compat', 'smallest'):
-            self._dtype_alias = datatype
-            return
+        # Numpy dtype comparison can fail in odd ways, check for aliases only if str
+        if isinstance(datatype, str):
+            # Static aliases
+            if datatype == 'mask':
+                datatype = 'u1'
+            # Dynamic aliases
+            elif datatype in ('compat', 'smallest'):
+                self._dtype_alias = datatype
+                return
 
         self._dtype_alias = None
         super().set_data_dtype(datatype)