Simplify the jupyter kernelspec setup

[davidorme]

Is your feature request related to a problem? Please describe.

We currently manually set up an explicit jupyter kernel specification used to specify the python kernel used in jupyter notebooks. This was a way of ensuring that the jupyter environment was consistent across developers, runners etc, despite the machine specific poetry environment names.

However poetry shell and poetry run now (or possibly always did) automatically adjust the 'jupyter kernels to point python3 to the active poetry virtual environment. This has the real advantage that the kernel is consistent with the current developer environment - the manual link can point to an outdated python version.

Describe the solution you'd like
Remove the manual setup of the pyrealm_python3 kernel spec and just use the python3 kernel across notebooks.