Log Message:
5.1:

Define Jupyter protocol version 5.2, resolving ambiguity of cursor_pos field in \ 
the presence of unicode surrogate pairs.

Add :meth:`Session.clone` for making a copy of a Session object without sharing \ 
the digest history. Reusing a single Session object to connect multiple sockets \ 
to the same IOPub peer can cause digest collisions.

Avoid global references preventing garbage collection of background threads.