Bump asv Python version
Got this error trying to run the asvs locally and bumping the Python version in asv.conf.json fixed it:

```
   STDOUT -------->
   Processing /Users/danielsaxton/pandas/asv_bench/env/a36d556a9d1611aba4092dc48036d261/project
       Preparing wheel metadata: started
       Preparing wheel metadata: finished with status 'done'
   STDERR -------->
   ERROR: Package 'pandas' requires a different Python: 3.6.10 not in '>=3.7.1'
```

Let me know if this is needed / correct in general or if it was just a thing with my machine.

cc @TomAugspurger 

Closes https://github.com/pandas-dev/pandas/issues/34677