updated template asv.conf.json to Python 3.8

- fix #3430
- automated benchmarks were updated (see MDAnalysis/benchmarks#11)

benchmarks/asv.conf.json

@@ -21,7 +21,7 @@
 
     // 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"],
+    "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
     // list indicates to just test against the default (latest)