Update CausalTree's base _tree module with sklearn 1.5.2 (#806)

* update inference.tree._tree with sklearn 1.5.2
* fix lint errors
* remove unused files from sklearn.tree._tree
* fix the sklearn version to be >= 1.5.2, 1.6
* update the readthedocs config to use py39

.readthedocs.yml

@@ -8,7 +8,7 @@ build:
     python: "miniconda3-4.7"
 
 conda:
-  environment: docs/environment-py38-rtd.yml
+  environment: docs/environment-py39-rtd.yml
 
 python:
   install:

causalml/inference/tree/_tree/__init__.py

@@ -1,4 +1,14 @@
 """
 This part of tree structures definition was initially borrowed from
-https://github.com/scikit-learn/scikit-learn/tree/1.0.2/sklearn/tree
+https://github.com/scikit-learn/scikit-learn/tree/1.5.2/sklearn/tree
 """
+
+"""Decision tree based models for classification and regression."""
+
+from ._classes import (
+    BaseDecisionTree,
+)
+
+__all__ = [
+    "BaseDecisionTree",
+]