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.
diff --git a/Lib/warnings.py b/Lib/warnings.py
index 8d46cd6..a88e7ba 100644
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -383,8 +383,8 @@
 # Module initialization
 _processoptions(sys.warnoptions)
 if not _warnings_defaults:
-    simplefilter("ignore", category=PendingDeprecationWarning, append=1)
-    simplefilter("ignore", category=ImportWarning, append=1)
+    for cls in (DeprecationWarning, PendingDeprecationWarning, ImportWarning):
+        simplefilter("ignore", category=cls, append=True)
     bytes_warning = sys.flags.bytes_warning
     if bytes_warning > 1:
         bytes_action = "error"