#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.
Issue #20546: Use specific asserts in int tests.
Issue #20555: Use specific asserts in urllib, httplib, ftplib, cgi, wsgiref tests.
Issue #20549: Use specific asserts in mailbox, smtplib and poplib tests.
Issue #20406: Use Python application icons for Idle window title bars.
Patch mostly by Serhiy Storchaka.
#14983: always add a line end after a MIME boundary marker.

This is more RFC compliant (see issue) and fixes a problem with
signature verifiers rejecting the part when signed.  There is some
amount of backward compatibility concern here since it changes
the output, but the RFC issue coupled with fixing the problem
with signature verifiers seems worth the small risk of breaking
code that depends on the current incorrect output.
Issue #20437: Fixed 43 potential bugs when deleting objects references.
#19906: clarify note in urllib docs.
issue12085: Use more Pythonic way to check _child_created.
_active shouldn't be cached, it set to None on shutdown.
Merge heads
Issue #19856: shutil.move() failed to move a directory to other directory
on Windows if source name ends with os.altsep.
Issue #20558: Improved implementation of error handling.
avoid name clash with posix_close (closes #20594)
Add references for Xcode and the Python Developer's Guide
to the internal Mac/README file.
Issue #19255: Clear error after failed PyDict_SetItem() on shutdown.
This silences a Coverity complain.
Issue #17671: Fixed a crash when use non-initialized io.BufferedRWPair.
Based on patch by Stephen Tu.
#19680: add back documentation for print and exec in the interactive help.
Issue #20605: Make test_socket getaddrinfo OS X segfault test more robust.
fix links to builtin repr function (closes #20573)
#20634: fix typo in IDLE README noticed by Saimadhav Heblikar.