changes python version back to 3.8

readthedocs.yml

@@ -17,7 +17,7 @@ formats:
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.8, 3.9
+  version: 3.8
   install:
     - requirements: doc/requirements.txt
     - method: pip