Restore default testRunner argument in unittest.main to None. Issue 6177.
diff --git a/Lib/unittest.py b/Lib/unittest.py
index 09c6ca9..2ad5de7 100644
--- a/Lib/unittest.py
+++ b/Lib/unittest.py
@@ -798,8 +798,10 @@
                                                in MyTestCase
 """
     def __init__(self, module='__main__', defaultTest=None,
-                 argv=None, testRunner=TextTestRunner,
+                 argv=None, testRunner=None,
                  testLoader=defaultTestLoader):
+        if testRunner is None:
+            testRunner = TextTestRunner
         if type(module) == type(''):
             self.module = __import__(module)
             for part in module.split('.')[1:]: