Try requiring scikit-learn<1.6 when xgboost is installed

pyproject.toml

@@ -52,6 +52,13 @@ lightgbm = [
 xgboost = [
     "xgboost>=2.0",
     "pyarrow>=4.0",
+    # As of 2024-12-10, the latest scikit-learn version (1.6.0) is incompatible
+    # with the latest xgboost version (2.1.3). scikit-learn 1.6.0 came out
+    # yesterday, 2024-12-09, so I'm guessing that this a temporary bug that
+    # will be resolved with an update to one of the two libraries sometime
+    # sooner rather than later. Until then, we can pin scikit-learn to < 1.6
+    # when using xgboost.
+    "scikit-learn<1.6.0",
 ]
 
 [project.scripts]