While it most likely won’t help at all in this case because you seem to be having additional issues with IPython and your system playing nicely, going forward you will have a better experience in modern Jupyter if you endeavor to use current best practices. Specifically, in regards to what I quote here, you should switch to using the magic variations of the install commands in all modern Jupyter, here that would be %pip install cesiumpy.
Use of an exclamation point with install commands doesn’t insure it installs to the correct environment. The magic variations of the install commands were added in 2019 to insure the installation occurs in the environment where the kernel is running that is backing the active .ipynb in which you run the installation, see here. More at top here about why the exclamation point for installations is deficient and can lead to issues.
(Additional tip: You can also sort of use this as a gauge to measure if documentation and advice are up-to-date with the current Jupyter ecosystem and be wary if it is not.)