Session._key_changed is deprecated in traitlets 4.1

[jakirkham]

Getting a DeprecationWarning in jupyter_client, which appears to be issued by traitlets. Points to this line as problematic. Warning included below.

/opt/conda3/lib/python3.6/site-packages/jupyter_client/session.py:371: DeprecationWarning: Session._key_changed is deprecated in traitlets 4.1: use @observe and @unobserve instead.
  def _key_changed(self):

[BjornFJohansson]

I see this as well. Should I update something?

========================================================= warnings summary =========================================================
tests/format_sequences.ipynb::Cell 0
  /home/bjorn/anaconda3/envs/bjorn38/lib/python3.8/site-packages/jupyter_client/manager.py:66: DeprecationWarning: KernelManager._kernel_spec_manager_changed is deprecated in traitlets 4.1: use @observe and @unobserve instead.
    def _kernel_spec_manager_changed(self):

-- Docs: https://docs.pytest.org/en/stable/warnings.html

11:32 $ conda list jupyter_client
# packages in environment at /home/bjorn/anaconda3/envs/bjorn38:
#
# Name                    Version                   Build  Channel
jupyter_client            6.1.7                      py_0    conda-forge
(bjorn38) ✔ ~/python_packages/pydna [master ↓·1|✚ 9…1⚑ 1] 
11:35 $ 

[SylvainCorlay]

yes this is a fairly simple change to make.

[rgbkrk]

I just noticed the original post has since been fixed but that _kernel_spec_manager_changed still needs it so I'm submitting a fix.