fix StringIO import for asv benchmarks (pandas-dev#25989)

Author: anmyachev

asv_bench/benchmarks/io/csv.py

@@ -4,7 +4,7 @@
 import numpy as np
 import pandas.util.testing as tm
 from pandas import DataFrame, Categorical, date_range, read_csv
-from pandas.compat import cStringIO as StringIO
+from io import StringIO
 
 from ..pandas_vb_common import BaseIO
 

asv_bench/benchmarks/io/excel.py

@@ -1,6 +1,6 @@
+from io import BytesIO
 import numpy as np
 from pandas import DataFrame, date_range, ExcelWriter, read_excel
-from pandas.compat import BytesIO
 import pandas.util.testing as tm