DeprecationWarning is now silent by default.

This was originally suggested by Guido, discussed on the stdlib-sig mailing
list, and given the OK by Guido directly to me. What this change essentially
means is that Python has taken a policy of silencing warnings that are only
of interest to developers by default. This should prevent users from seeing
warnings which are triggered by an application being run against a new
interpreter before the app developer has a chance to update their code.

Closes issue #7319. Thanks to Antoine Pitrou, Ezio Melotti, and Brian Curtin
for helping with the issue.

Lib/test/test_exceptions.py

@@ -309,6 +309,7 @@ class ExceptionTests(unittest.TestCase):
         # BaseException.__init__ triggers a deprecation warning.
         exc = BaseException("foo")
         with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('default')
             self.assertEquals(exc.message, "foo")
         self.assertEquals(len(w), 1)
         self.assertEquals(w[0].category, DeprecationWarning)