add the readthedocs environment file for python 3.9

docs/environment-py39-rtd.yml

@@ -0,0 +1,32 @@
+name: causalml-rtd-py39
+channels:
+  - conda-forge
+  - defaults
+dependencies:
+  - pip=24.0
+  - python=3.9
+  - pandoc
+  - sphinx
+  - sphinx_rtd_theme
+  - sphinxcontrib-bibtex<2.0.0
+  - nbsphinx
+  - pip:
+    - cython==0.29.34
+    - dill==0.3.8
+    - importlib-metadata==8.5.0
+    - joblib==1.4.0
+    - lightgbm==4.5.0
+    - matplotlib==3.9.2
+    - multiprocess==0.70.16
+    - numba==0.60.0
+    - numpy==1.26.4
+    - pandas==2.2.2
+    - pygam==0.9.1
+    - pyro-api==0.1.2
+    - pyro-ppl==1.9.1
+    - scikit-learn==1.5.2
+    - scipy==1.11.4
+    - seaborn==0.13.2
+    - shap==0.46.0
+    - statsmodels==0.14.2
+    - xgboost==2.1.3