sip_im_session.py: updated for latest eventlet (589)

darcs-hash:20090120144016-b2873-288785514abbcfd12024b589e3140baad8d52d67
Make sure we don't try to patch something that is already in the PJSIP SVN tree

darcs-hash:20090120153026-58103-844c28c78e8562a4c73c94e13658a74f846f6eb9