Small typing fix (#6159)

Author: Illviljan

xarray/core/concat.py

@@ -6,6 +6,7 @@
     Hashable,
     Iterable,
     List,
+    Literal,
     Optional,
     Set,
     Tuple,
@@ -14,7 +15,6 @@
 )
 
 import pandas as pd
-from typing_extensions import Literal
 
 from . import dtypes, utils
 from .alignment import align