Bump asv Python version (pandas-dev#35798)

* Bump asv Python version

* 3.7.1 -> 3.8

Author: dsaxton

asv_bench/asv.conf.json

@@ -26,7 +26,7 @@
     // The Pythons you'd like to test against.  If not provided, defaults
     // to the current version of Python used to run `asv`.
     // "pythons": ["2.7", "3.4"],
-    "pythons": ["3.6"],
+    "pythons": ["3.8"],
 
     // The matrix of dependencies to test.  Each key is the name of a
     // package (in PyPI) and the values are version numbers.  An empty