Move test.test_support.catch_warning() to the warnings module, rename it
catch_warnings(), and clean up the API.

While expanding the test suite, a bug was found where a warning about the
'line' argument to showwarning() was not letting functions with '*args' go
without a warning.

Closes issue 3602.
Code review by Benjamin Peterson.
diff --git a/Lib/mimetools.py b/Lib/mimetools.py
index 097eda4..fc5a2a5 100644
--- a/Lib/mimetools.py
+++ b/Lib/mimetools.py
@@ -2,11 +2,12 @@
 
 
 import os
+import sys
 import tempfile
-from test.test_support import catch_warning
-from warnings import filterwarnings
-with catch_warning(record=False):
-    filterwarnings("ignore", ".*rfc822 has been removed", DeprecationWarning)
+from warnings import filterwarnings, catch_warnings
+with catch_warnings(record=False):
+    if sys.py3kwarning:
+        filterwarnings("ignore", ".*rfc822 has been removed", DeprecationWarning)
     import rfc822
 
 from warnings import warnpy3k