[python-package] remove unused imports (#5776)

.gitignore

@@ -51,6 +51,7 @@ Debug
 #.Rbuildignore
 R-package.Rproj
 *.cache*
+.mypy_cache/
 # java
 java/xgboost4j/target
 java/xgboost4j/tmp

doc/sphinx_util.py

@@ -2,7 +2,6 @@
 """Helper utility function for customization."""
 import sys
 import os
-import docutils
 import subprocess
 
 READTHEDOCS_BUILD = (os.environ.get('READTHEDOCS', None) is not None)

python-package/xgboost/__init__.py

@@ -5,8 +5,6 @@ Contributors: https://github.com/dmlc/xgboost/blob/master/CONTRIBUTORS.md
 """
 
 import os
-import sys
-import warnings
 
 from .core import DMatrix, DeviceQuantileDMatrix, Booster
 from .training import train, cv

tests/benchmark/benchmark_linear.py

@@ -1,5 +1,5 @@
 #pylint: skip-file
-import sys, argparse
+import argparse
 import xgboost as xgb
 import numpy as np
 from sklearn.datasets import make_classification