Documentation: Add i3msg-python to IPC libraries list

[Ceryn]

Could you add my Python interface to the i3 IPC libraries list?

Link to the project:
https://github.com/Ceryn/i3msg-python

Link to the i3 IPC libraries list:
https://i3wm.org/docs/ipc.html#_see_also_existing_libraries

Link to Reddit announcement thread:
https://www.reddit.com/r/i3wm/comments/7m8hfr/psa_i3msgpython_a_minimal_python_interface_to_the/

[i3bot]

I don’t see a link to logs.i3wm.org. Did you follow https://i3wm.org/docs/debugging.html? (In case you actually provided a link to a logfile, please ignore me.)

[i3bot]

I don’t see a version number. Could you please copy & paste the output of i3 --version into this issue?

[Airblader]

I don't see a reason why not. Please just send a PR that adds it. :-)