Fixed py36-only syntax [ci skip] (#22167)

Closes #22162

[ci skip]

Author: TomAugspurger

asv_bench/benchmarks/reshape.py

@@ -141,7 +141,7 @@ class GetDummies(object):
 
     def setup(self):
         categories = list(string.ascii_letters[:12])
-        s = pd.Series(np.random.choice(categories, size=1_000_000),
+        s = pd.Series(np.random.choice(categories, size=1000000),
                       dtype=pd.api.types.CategoricalDtype(categories))
         self.s = s