slight fix for readthedocs config

.readthedocs.yaml

@@ -9,12 +9,8 @@ version: 2
 sphinx:
    configuration: docs/src/conf.py
 
-# Optionally build your docs in additional formats such as PDF
-formats:
-   - pdf
-
 # Optionally set the version of Python and requirements required to build your docs
 python:
-   version: "3.9"
+   version: "3.8"
    install:
    - requirements: docs/src/requirements.txt