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