updated rtd yaml regarding issue:readthedocs/sphinx_rtd_theme#923

.readthedocs.yaml

@@ -9,11 +9,14 @@ version: 2
 build:
   os: ubuntu-20.04
   tools:
-    python: "3.8"
+    python: "3"
     # You can also specify other tool versions:
     # nodejs: "16"
     # rust: "1.55"
     # golang: "1.17"
+  jobs:
+    post_create_environment:
+      - python -m pip install sphinx_rtd_theme
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx: