Revert pushing RTD to Python 3.10

It causes unexpected breakage because RTD does not offer Python 3.9 or 3.10 runners. This might be a problem when introducing new features from later Python versions

.readthedocs.yml

@@ -1,7 +1,7 @@
 version: 2
 
 python:
-   version: 3.10
+   version: 3.8
    install:
        - requirements: docs/requirements.txt