Ignore ruff/Pyflakes rule F401 more precisely (pydata#10369)

F401 imported but unused

The `unravel_index` import was added by cd6065e / pydata#5571.

Author: DimitriPapadopoulos

xarray/core/duck_array_ops.py

@@ -17,11 +17,11 @@
 
 import numpy as np
 import pandas as pd
-from numpy import (  # noqa: F401
+from numpy import (
     isclose,
     isnat,
     take,
-    unravel_index,
+    unravel_index,  # noqa: F401
 )
 from pandas.api.types import is_extension_array_dtype