Update Python version used on readthedocs

Python 3.6 is EOL and causes issues (lack of dataclasses breaks sphinx.ext.autodoc).

I'm not sure what version is appropriate.  ReadTheDocs themselves seem to have [defaulted to Python 3.8](readthedocs/readthedocs.org#7421), so that's what I went with.

Author: mgedmin

readthedocs.yml

@@ -9,6 +9,6 @@ build:
 formats: all
 
 python:
-  version: 3.6
+  version: 3.8
   install:
     - requirements: docs/requirements.txt