[python-package] Remove dead Python 2 guard and sklearn.cross_validation fallback

cmake/FindOpenMPMacOS.cmake

@@ -1,13 +1,28 @@
 # Find OpenMP library on MacOS
-# Automatically handle locating libomp from the Homebrew package manager
+# Prefer libomp from the active Conda environment and fall back to Homebrew.
 
 # lint_cmake: -package/consistency
 
 macro(find_openmp_macos)
   if(NOT APPLE)
     message(FATAL_ERROR "${CMAKE_CURRENT_FUNCTION}() must only be used on MacOS")
   endif()
-  find_package(OpenMP)
+
+  if(DEFINED ENV{CONDA_PREFIX} AND EXISTS "$ENV{CONDA_PREFIX}/lib/libomp.dylib")
+    set(CONDA_LIBOMP_PREFIX "$ENV{CONDA_PREFIX}")
+    set(OpenMP_C_FLAGS
+      "-Xpreprocessor -fopenmp -I${CONDA_LIBOMP_PREFIX}/include")
+    set(OpenMP_CXX_FLAGS
+      "-Xpreprocessor -fopenmp -I${CONDA_LIBOMP_PREFIX}/include")
+    set(OpenMP_C_LIB_NAMES omp)
+    set(OpenMP_CXX_LIB_NAMES omp)
+    set(OpenMP_omp_LIBRARY ${CONDA_LIBOMP_PREFIX}/lib/libomp.dylib)
+    find_package(OpenMP)
+  endif()
+
+  if(NOT OpenMP_FOUND)
+    find_package(OpenMP)
+  endif()
   if(NOT OpenMP_FOUND)
     # Try again with extra path info. This step is required for libomp 15+ from Homebrew,
     # as libomp 15.0+ from brew is keg-only
@@ -106,6 +121,7 @@ function(patch_openmp_path_macos target target_default_output_name)
   )
   # Add RPATH entries to ensure the loader looks in the following, in the following order:
   #
+  #   - $CONDA_PREFIX/lib             (when building inside a Conda environment)
   #   - /opt/homebrew/opt/libomp/lib  (where 'brew install' / 'brew link' puts libomp.dylib)
   #   - ${__OpenMP_LIBRARY_DIR}       (wherever find_package(OpenMP) found OpenMP at build time)
   #
@@ -114,11 +130,15 @@ function(patch_openmp_path_macos target target_default_output_name)
   execute_process(COMMAND brew --prefix libomp
                   OUTPUT_VARIABLE HOMEBREW_LIBOMP_PREFIX
                   OUTPUT_STRIP_TRAILING_WHITESPACE)
+  set(__OPENMP_RPATHS "${HOMEBREW_LIBOMP_PREFIX}/lib;${__OpenMP_LIBRARY_DIR}")
+  if(DEFINED ENV{CONDA_PREFIX} AND EXISTS "$ENV{CONDA_PREFIX}/lib/libomp.dylib")
+    set(__OPENMP_RPATHS "$ENV{CONDA_PREFIX}/lib;${__OPENMP_RPATHS}")
+  endif()
   set_target_properties(
     ${target}
     PROPERTIES
       BUILD_WITH_INSTALL_RPATH TRUE
-      INSTALL_RPATH "${HOMEBREW_LIBOMP_PREFIX}/lib;${__OpenMP_LIBRARY_DIR}"
+      INSTALL_RPATH "${__OPENMP_RPATHS}"
       INSTALL_RPATH_USE_LINK_PATH FALSE
   )
 endfunction()

ops/pipeline/build-python-wheels-macos.sh

@@ -36,6 +36,12 @@ else
     exit 2
 fi
 
+# Prefer Conda's OpenMP runtime for wheel builds on macOS. Homebrew's libomp can
+# pull in Homebrew LLVM's libunwind, which raises the effective deployment target
+# of the repaired wheel.
+conda install -y llvm-openmp
+export CMAKE_PREFIX_PATH="${CONDA_PREFIX}${CMAKE_PREFIX_PATH:+:${CMAKE_PREFIX_PATH}}"
+
 # Tell delocate-wheel to not vendor libomp.dylib into the wheel
 export CIBW_REPAIR_WHEEL_COMMAND_MACOS="delocate-wheel --require-archs {delocate_archs} -w {dest_dir} -v {wheel} --exclude libomp.dylib"
 

python-package/xgboost/compat.py

@@ -16,8 +16,6 @@
     import pandas as pd
     import pyarrow as pa
 
-assert sys.version_info[0] == 3, "Python 2 is no longer supported."
-
 
 def py_str(x: bytes | None) -> str:
     """convert c string back to python string"""
@@ -42,11 +40,7 @@ def lazy_isinstance(instance: Any, module: str, name: str) -> bool:
     from sklearn.base import BaseEstimator as XGBModelBase
     from sklearn.base import ClassifierMixin as XGBClassifierBase
     from sklearn.base import RegressorMixin as XGBRegressorBase
-
-    try:
-        from sklearn.model_selection import StratifiedKFold as XGBStratifiedKFold
-    except ImportError:
-        from sklearn.cross_validation import StratifiedKFold as XGBStratifiedKFold
+    from sklearn.model_selection import StratifiedKFold as XGBStratifiedKFold
 
     # sklearn.utils Tags types can be imported unconditionally once
     # xgboost's minimum scikit-learn version is 1.6 or higher