ENH: support for `PySide6` in `%gui` (#13864) Addresses #13859 Changes here parallel the changes in [ipykernel](https://github.com/ipython/ipykernel/pull/1054), i.e. prefer `PyQt` over `PySide` and allow requesting explicit versions (e.g. 'qt5') with one difference (see below). I believe that, eventually, the Qt importing logic should be all here, and `ipykernel` should defer to`ipython`. I chose not to do that at this time since it would mean the latest `ipykernel` would require the latest `IPython`; I'm happy to discuss further. The only difference between `IPython` and `ipykernel`, after these two pull requests, is that, in `IPython`, it's not possible to explicitly request an event loop for Qt4. This is because an alias exists [here](https://github.com/ipython/ipython/blob/5409de68d87ddd073a35111aca0cb8360ff63ca8/IPython/terminal/pt_inputhooks/__init__.py#L5) which effectively makes "qt4" be "the latest Qt available". I did not remove the alias because I don't know the history behind it.