Re-enable `check-python` step in Jupyter Releaser

[dlqqq]

Problem

#1280 helped ensure that jupyter-ai==X.Y.Z always requires jupyter-ai-magics>=X.Y.Z, preventing future issues about the two packages being out of sync.

However, we had to disable the check-python step as a workaround to jupyter-server/jupyter_releaser#499. In summary, Jupyter Releaser doesn't support monorepos with packages that require the exact same version of another package due to the check-python step.

Proposed Solution

1. Identify & implement a fix for the upstream issue in Jupyter Releaser
2. Re-enable the check-python step in our project's Jupyter Relaser config.