fix: update .readthedocs.yaml

Attempt to fix the build. It is unclear why this is failing,
so simply attempting to match the following as closely
as possible:
https://docs.readthedocs.io/en/stable/config-file/v2.html

The failure no longer has an error (which used to be around
missing os), and the raw output is missing the call:
> cat .readthedocs.yaml

It's unclear what is breaking.

.readthedocs.yaml

@@ -15,6 +15,5 @@ sphinx:
   configuration: docs/conf.py
 
 python:
-  version: 3.8
   install:
     - requirements: requirements/doc.txt