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/mimetools.py

@@ -5,7 +5,7 @@ import os
 import sys
 import tempfile
 from warnings import filterwarnings, catch_warnings
-with catch_warnings(record=False):
+with catch_warnings():
     if sys.py3kwarning:
         filterwarnings("ignore", ".*rfc822 has been removed", DeprecationWarning)
     import rfc822