Refuse to install event loop hooks when not using `prompt_toolkit` (#14132) Without this, `%gui` is effectively a no-op but the user thinks it works. For example. If running `ipython`: ``` In [1]: import matplotlib; matplotlib.use('QtAgg'); from matplotlib import pyplot; pyplot.ion(); pyplot.plot([1, 2, 3, 4]) Installed qt6 event loop hook. Out[1]: [<matplotlib.lines.Line2D at 0x1ba2f59d2a0>] ``` The window appears and responds as expected. If running `ipython --simple-prompt`, the user would see the same output, when in fact no event loop hook was installed since it's not supported without `prompt_toolkit`. The resulting Qt window is unresponsive because the event loop is not running, i.e. with `--simple-prompt`, Qt windows should block (but `pyplot` doesn't/can't know to do that) With this PR, the user will see: ``` In [1]: import matplotlib; matplotlib.use('QtAgg'); from matplotlib import pyplot; pyplot.ion(); pyplot.plot([1, 2, 3, 4]) Cannot install event loop hook for "qt" when running with `--simple-prompt`. NOTE: Tk is supported natively; use Tk apps and Tk backends with `--simple-prompt`. Out[1]: [<matplotlib.lines.Line2D at 0x170be0c0310>] ``` They'll still get an unresponsive Qt window, but they'll at least be told this can't work (while anything using Tk will work just fine).