gtk/GTKAgg matplotlib backend is not available

[ianthomas23]

Using the latest IPython (8.22.2) and Matplotlib (3.8.3) the list of available IPython backends is

In [1]: %matplotlib --list
Available matplotlib backends: ['tk', 'gtk', 'gtk3', 'gtk4', 'wx', 'qt4', 'qt5', 'qt6', 'qt', 'osx', 'nbagg', 'webagg', 'notebook', 'agg', 'svg', 'pdf', 'ps', 'inline', 'ipympl', 'widget']

which includes 'gtk'. But there is no such backend available in Matplotlib:

In [1]: %matplotlib gtk
<snip>
ValueError: Key backend: 'gtkagg' is not a valid value for backend; supported values are ['GTK3Agg', 'GTK3Cairo', 'GTK4Agg', 'GTK4Cairo', 'MacOSX', 'nbAgg', 'QtAgg', 'QtCairo', 'Qt5Agg', 'Qt5Cairo', 'TkAgg', 'TkCairo', 'WebAgg', 'WX', 'WXAgg', 'WXCairo', 'agg', 'cairo', 'pdf', 'pgf', 'ps', 'svg', 'template']

I think it was removed in 2018 (matplotlib/matplotlib#10426) so I assume there has been no real-world use of it for a while.

I think it should be removed from the list of allowed backends in IPython. However, I don't think any action is necessary now as I will deal with this as part of the wider change to move the matplotlib backend resolution from IPython to Matplotlib (#14311).