TST: Test CategoricalIndex in test_is_categorical (#16243)

Follow-up to gh-16237.

Author: gfyoung

pandas/core/dtypes/common.py

@@ -205,13 +205,15 @@ def is_categorical(arr):
     >>> is_categorical([1, 2, 3])
     False
 
-    Categoricals and Series Categoricals will return True.
+    Categoricals, Series Categoricals, and CategoricalIndex will return True.
 
     >>> cat = pd.Categorical([1, 2, 3])
     >>> is_categorical(cat)
     True
     >>> is_categorical(pd.Series(cat))
     True
+    >>> is_categorical(pd.CategoricalIndex([1, 2, 3]))
+    True
     """
 
     return isinstance(arr, ABCCategorical) or is_categorical_dtype(arr)

pandas/tests/dtypes/test_common.py

@@ -158,6 +158,7 @@ def test_is_categorical():
     cat = pd.Categorical([1, 2, 3])
     assert com.is_categorical(cat)
     assert com.is_categorical(pd.Series(cat))
+    assert com.is_categorical(pd.CategoricalIndex([1, 2, 3]))
 
     assert not com.is_categorical([1, 2, 3])