Compatibility fixes for ipykernel 6

[mdickinson]

This PR contains some minor compatibility fixes for ipykernel versions 6.0.0 and later. There's still significant work to do before we have full compatibility - this is really just clearing away some of the weeds so that we can see the real issues.

Details

• We use capture_fd_output=False when creating the IPKernelApp. This prevents the kernel application from setting up threads for stdout and stderr redirection. Those threads have been causing shutdown issues.
• In log_connection_info, we set either the _ports traitlet or the ports traitlet; whichever works first (this trait was renamed at some point in the ipykernel history)
• We retrieve the history manager earlier so that we can shut its bits down - the new ipykernel sets the history manager trait to None when atexit_operations is called, so that we can no longer retrieve the history manager after calling atexit_operations.
• the most recent IPython has deleted the IPython.utils.io.stdout and IPython.utils.io.stderr attributes; update our cleanup code to allow for this case

Related upstream issues:

• https://github.com/ipython/ipykernel/issues/867
• https://github.com/ipython/ipykernel/issues/868

[mdickinson]

This PR is ready for review.

[corranwebster]

One additional question - does this need any changes to the setup.py optional dependencies for ipython?

[mdickinson]

does this need any changes to the setup.py optional dependencies for ipython

Unfortunately not: it moves us a bit towards being able to relax the current version constraints, but we don't get all the way there.