a memory leak in the Python library

[Chris Brannon]

It turns out that SSIPClient objects can't be garbage-collected, when
they use the default callback handler.  I'll explain the problem, and
I'll send a patch later.

Say that we have an SSIPClient object named sc.
sc._conn refers to some _SSIPConnection object, which we'll name
our_conn.  The default callback for our_conn is one of sc's bound
methods: sc._callback_handler.
When sc.close() returns successfully, sc._conn still refers to
our_conn, and our_conn._callback still refers to one of sc's methods.
It's a circular reference, and sc can't be garbage-collected.

The solution is to simply execute
del self._conn
at the end of SSIPClient.close().  I think this is completely safe.

-- Chris