Update the embedded copy of the expat XML parser to 2.1.0.  It brings
with it a vareity of bug fixes, both security and behavior.  See
http://www.libexpat.org/ for the list.

NOTE: I already backported the expat hash randomization fix in March.

Fixes issue #14340.
Fix the test and remove trailing dots on Windows for issue #6972.
merge heads
Issue #15881: Fixed atexit hook in multiprocessing.
Preserve backslashes in malicious zip files for testing issue #6972.
Issue #17034: Use Py_CLEAR() in stringobject.c.
Fix test for issue #6972.
Issue 16398: Use memcpy() in deque.rotate().
Back out fix for issue #13886; it introduced a new bug in interactive readline use.
Issue #15633: httplib.HTTPResponse is now mark closed when the server sends less than the advertised Content-Length.
Issue #16698: Skip posix test_getgroups when built with OS X
deployment target prior to 10.6.