[python][tests] unused and missing imports (#3023)

tests/c_api_test/test_.py

 # coding: utf-8
 import ctypes
 import os
-import sys
 
 from platform import system
 

tests/python_package_test/test_engine.py

@@ -1857,7 +1857,6 @@ class TestEngine(unittest.TestCase):
         self.assertEqual(len(set([iter_valid1_l1, iter_valid1_l2, iter_valid2_l1, iter_valid2_l2])), 4)
         iter_min_l1 = min([iter_valid1_l1, iter_valid2_l1])
         iter_min_l2 = min([iter_valid1_l2, iter_valid2_l2])
-        iter_min = min([iter_min_l1, iter_min_l2])
         iter_min_valid1 = min([iter_valid1_l1, iter_valid1_l2])
 
         iter_cv_l1 = 4

tests/python_package_test/test_sklearn.py

@@ -4,6 +4,7 @@ import joblib
 import math
 import os
 import unittest
+import warnings
 
 import lightgbm as lgb
 import numpy as np