Creates and register the comm target when it does not exist

[JohanMabille]

Fixes #672

[martinRenou]

Does it work to re-register the comm target after the fact?

import comm

# This creates a dummy callback
comm.create_comm(target_name="foo")

def handle_comm_opened(*args, **kwargs):
    # TODO handle comm open from front-end
    pass


comm_manager = comm.get_comm_manager()
if comm_manager is not None:
    comm_manager.register_target("foo", handle_comm_opened)

[JohanMabille]

It is supposed to work because the registration ends here. However, executing this coe in a single cell results in a kernel crash for another reason (the GIL is not handled correctly). Splitting the code in the two following cells:

import comm

# This creates a dummy callback
comm.create_comm(target_name="foo")

and

def handle_comm_opened(*args, **kwargs):
    # TODO handle comm open from front-end
    pass


comm_manager = comm.get_comm_manager()
if comm_manager is not None:
    comm_manager.register_target("foo", handle_comm_opened)

makes it work.

The issue seems to come from the call of comm_close received after creating the comm. If we have any python left to execute from the cell, it crashes. I'll investigate and fix that one in a dedicated PR.

[JohanMabille]

Looks like the issue is in xeus itself, so let's release xeus-python to fix the most common issue