pin python=3.13 for RTD building (#24)

Author: jeremyleung521

devtools/conda-envs/environment-rtd.yml

@@ -3,7 +3,7 @@ channels:
     - conda-forge
     - defaults
 dependencies:
-    - python
+    - python=3.13
     - westpa>=2022.03
     - scikit-learn
     - tqdm