test: Proactively delete potential filehandle refs

Author: effigies

nibabel/tests/test_proxy_api.py

@@ -166,6 +166,10 @@ def validate_array_interface_with_dtype(self, pmaker, params):
                 assert_dt_equal(out.dtype, np.dtype(dtype))
                 # Shape matches expected shape
                 assert out.shape == params['shape']
+                del out
+            del direct
+
+        del orig
 
         if context is not None:
             context.__exit__()