Fix kernel failing to start due to wrong typing

thedo_shutdown method throws the following error:

TypeError: 'type' object is not subscriptable

, which prevents the kernel from starting.

This happens because dict[str, str] is used. Instead, Dict should be used:

Dict[str, str]

Note:

I have tested this on Ubuntu 20.04.5 with the following packages:

IPython          : 8.8.0
ipykernel        : 6.20.2
ipywidgets       : 7.7.2
jupyter_client   : 7.0.6
jupyter_core     : 5.1.3
jupyter_server   : 1.11.1
jupyterlab       : 3.1.18
nbclient         : 0.5.4
nbconvert        : 7.2.7
nbformat         : 5.7.3
notebook         : 6.5.2
qtconsole        : 4.7.4
traitlets        : 5.8.1