warnings.catch_warnings() now returns a list or None instead of the custom

WarningsRecorder object. This makes the API simpler to use as no special object
must be learned.

Closes issue 3781.
Review by Benjamin Peterson.

Lib/test/test_re.py

@@ -1,7 +1,7 @@
 import sys
 sys.path = ['.'] + sys.path
 
-from test.test_support import verbose, run_unittest, catch_warning
+from test.test_support import verbose, run_unittest
 import re
 from re import Scanner
 import sys, os, traceback
@@ -447,7 +447,7 @@ class ReTests(unittest.TestCase):
         self.pickle_test(cPickle)
         # old pickles expect the _compile() reconstructor in sre module
         import warnings
-        with catch_warning():
+        with warnings.catch_warnings():
             warnings.filterwarnings("ignore", "The sre module is deprecated",
                                     DeprecationWarning)
             from sre import _compile