Tighten tpying restriction of nbformat.ICell
Tighten tpying restriction of nbformat.ICell

By narrowing ICell to not include the base class, type guards can
effectively differentiate between the different cell types based on the
`cell.cell_type` field.

eskirk/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,charnpreetsingh185/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab,eskirk/jupyterlab,eskirk/jupyterlab