[IPython-User] Plugin system deprecated

[Thomas Kluyver]

Hi all,

We're planning to drop the plugin system (extensions inheriting from
the Plugin class), as we don't think anyone is using it. If you know
of any extensions using the plugin system, please let us know.

https://github.com/ipython/ipython/pull/2291

The extension system will still exist, and you're encouraged to use
that for new and existing extensions.

Thanks,
Thomas