remove unused imports (#5882)

Author: saschanaz

tests/parallel_runner.py

@@ -1,11 +1,8 @@
 from __future__ import print_function
 import multiprocessing
 import os
-import pickle
 import subprocess
 import sys
-import time
-import traceback
 import unittest
 
 try:
@@ -59,7 +56,7 @@ def reversed_tests(self):
     tests = []
     for test in self:
       tests.append(test)
-    tests.sort(key=lambda test: str(test))
+    tests.sort(key=str)
     return tests[::-1]
 
   def init_processes(self, test_queue):