matplotlib_inline dependency missing in nbdev_test

# matplotlib_inline dependency missing

After migrating to nbdev2, running `nbdev_test` on my Jupyter notebooks with `matplotlib` plots produced the following error:

ModuleNotFoundError: No module named 'matplotlib_inline'

This error only occurs when running `nbdev`_test or `nbdev_prepare`. The notebooks render fine. After some searching online I found that there is a separate python package on pypi: `matplotlib-inline`. See: https://pypi.org/project/matplotlib-inline/

The documentation says: *Note that in current versions of JupyterLab and Jupyter Notebook, the explicit use of the %matplotlib inline directive is not needed anymore, though other third-party clients may still require it.*

After `pip install matplotlib-inline` the error no longer occurred.

Would it be possible to add this requirement to nbdev? It seems to me that this would prevent similar problems with other users.

Thanks,

Frank

Comienza localizando los metadatos de dependencias que usan nbdev_test y nbdev_prepare y, después, reproduce el ModuleNotFoundError con un notebook que contenga un gráfico de matplotlib. El issue estará terminado cuando instalar nbdev proporcione matplotlib-inline y ambos puntos de entrada se ejecuten sin este error de módulo faltante.

Escrito por el modelo de indexación a partir del texto del issue.