#11700: proxy object close methods can now be called multiple times

This makes them work like the close provided by regular file objects.  This
patch also backports the close-the-underlying-file code for _ProxyFile objects
that was introduced along with context manager support in the 3.x branch.
diff --git a/Misc/NEWS b/Misc/NEWS
index 5fa4178..2e33e7f 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -16,6 +16,9 @@
 Library
 -------
 
+- Issue #11700: mailbox proxy object close methods can now be called multiple
+  times without error, and _ProxyFile now closes the wrapped file.
+
 - Issue #12133: AbstractHTTPHandler.do_open() of urllib.request closes the HTTP
   connection if its getresponse() method fails with a socket error. Patch
   written by Ezio Melotti.