#20013: don't raise socket error when selected mailbox deleted.

I'm checking this in without a test because not much of this code
is tested and I don't have time to work up the necessary extensions
to the existing test framework.

The patch itself was tested by the person who reported the bug.
diff --git a/Misc/NEWS b/Misc/NEWS
index 9520fa4..9479ded 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -42,6 +42,10 @@
 Library
 -------
 
+- Issue #20013: Some imap servers disconnect if the current mailbox is
+  deleted, and imaplib did not handle that case gracefully.  Now it
+  handles the 'bye' correctly.
+
 - Issue #20426: When passing the re.DEBUG flag, re.compile() displays the
   debug output every time it is called, regardless of the compilation cache.