New dependency for IPython: matplotlib-inline

Hi all,

For the past few years there have been some complex interactions between IPython and matplotlib in order to allow showing inline figures. This lead to sometime circular dependency or unclear responsibility about wether some behavior should be in matplotlib or IPython.

Those functionalities have been extracted into their own package on which IPython depends since this pull-request.

matplotlib-inline is already available on pypi and conda-forge; though if you are a downstream packager, or need security reviews of new packages for internal use in secure networks, this is a heads up that this dependency will now be necessary.

We also – of course – welcome any bug reports due to this change.