ipython: remove plugin (moving to separate package)

[dgw]

IPython as a dependency isn't as fiddly as aspell (see discussion in #1291, #1640, #1642), but it's still unnecessary install-bloat that applies only to a small portion of the user base. ipython and spellcheck should be the only exceptions to the package-ization plan outlined in #1291, which targets most removals for Sopel 8 (our next next major release).

Removed plugin file, requirements, test exclusions (???), and contrib script for hiding warnings generated by IPython. (I don't know if that contrib script even still does anything.)

New package will live at sopel-irc/sopel-ipython, and will be published on PyPI as sopel-ipython. @sopel-irc/rockstars, I'd like your opinions on whether the new package should be added as a requirement for sopel, as dictated by the transition plan in #1291. I'm on the fence, but given its low utility and the ease of reinstalling the plugin if desired, I'm leaning toward not bothering.

Merging logging changes like #1678 may require applying the ipython.py changes from 30a597f e18681b (@Exirel's sometimes-unnecessary rebasing strikes again!) to the new package.

[Exirel]

As far as I'm concerned, this is probably not something Sopel should ship with by default.

It's a niche feature, and I believe the Sopel contributors have been doing a great job at documenting how to install, and where to find plugins (for example, by being present on IRC to help newcomers). As long as we keep doing such efforts, I think it's safe to assume it's not necessary to add it to Sopel's dependencies.

[dgw]

The plugin will be installable shortly from https://pypi.org/project/sopel-ipython/ (or see the repo).
I forgot to publish a package last night. 😅

Note: As PyPI does not have any release of Sopel 7.x available yet, this requires an existing "editable" install of Sopel @ 40162a4 or newer. pip will not be able to satisfy the sopel>=7.0 dependency on its own until we begin publishing alpha/beta/RC versions.