change Python version from 3.11 to 3.9

.readthedocs.yaml

@@ -9,23 +9,22 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.11"
+    python: "3.9"  # "3.11" fails with d3rlpy==1.1.1
     # You can also specify other tool versions:
     # nodejs: "19"
     # rust: "1.64"
     # golang: "1.19"
   jobs:
     post_install:
       - pip install Cython numpy
-      - pip install d3rlpy>=1.1.1
+      - pip install d3rlpy==1.1.1
       - pip install scipy>=1.10.1
       - pip install numpy>=1.22.4
       - pip install pandas>=1.5.3
       - pip install scikit-learn>=1.0.2
       - pip install matplotlib>=3.7.1
       - pip install seaborn>=0.11.2
       - pip install torch>=2.0.0
-      - pip install d3rlpy==1.1.1
       - pip install gym>=0.26.2
       - pip install gymnasium>=0.28.1
       - pip install hydra-core>=1.3.2