Missing gtk4 loop integration

[anntzer]

In jupyter console, trying to show a gtk4agg matplotlib plot raises

UsageError: Invalid GUI request 'gtk4', valid ones are:dict_keys(['inline', 'nbagg', 'webagg', 'notebook', 'ipympl', 'widget', None, 'qt', 'qt5', 'qt6', 'wx', 'tk', 'gtk', 'gtk3', 'osx', 'asyncio'])

(Also, I note that both gtk and gtk3 are deprecated since #1036, is there a reason why? -- gtk (if that means the old gtkagg backend(?)) can probably be removed wholesale as that's Py2-only, but gtk3 should still work.)

[blink1073]

Hi @anntzer, I deprecated them because they were unmaintained, but didn't want to just yank them.

[anntzer]

I suspect they still see some usage in the wild (I have no numbers to provide though).
Also note that AFAICT the deprecation warning emitted by the kernel is not displayed to the end user, so I suspect many don't know about the deprecation.