Update jupyter-client requirement and improve some kernel manager stuff

[PekkaSavolainen]

• Update requirements to allow latest jupyter-client
• Add/update some kernel manager related stuff

Re issue spine-tools/Spine-Toolbox#2154
Re issue spine-tools/Spine-Toolbox#1495

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes in Toolbox repo have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black
• Unit tests pass

[codecov-commenter]

Codecov Report

Patch coverage: 72.11% and project coverage change: +0.29 🎉

Comparison is base (12174eb) 68.34% compared to head (2310abb) 68.64%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   68.34%   68.64%   +0.29%     
==========================================
  Files          47       47              
  Lines        3794     3878      +84     
  Branches      731      757      +26     
==========================================
+ Hits         2593     2662      +69     
- Misses       1044     1059      +15     
  Partials      157      157              

Impacted Files	Coverage Δ
...ne/execution_managers/conda_kernel_spec_manager.py	15.74% <0.00%> (ø)
...ine/execution_managers/kernel_execution_manager.py	63.68% <22.22%> (-3.99%)	⬇️
spine_engine/project_item/connection.py	83.68% <100.00%> (+0.40%)	⬆️
spine_engine/project_item/executable_item_base.py	74.39% <100.00%> (+0.97%)	⬆️
spine_engine/spine_engine.py	78.36% <100.00%> (+0.04%)	⬆️
spine_engine/utils/helpers.py	72.65% <100.00%> (+6.83%)	⬆️

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