Document that %gui qt currently forces Qt5

Currently, when users select qt with %gui qt or %matplotlib qt, IPython forces matplotlib to switch to Qt5Agg first, before matplotlib's qt_compat.py kicks in (which, if run on its own, can actually pick up the right Qt bindings installed).

ipython/IPython/core/pylabtools.py

Line 26 in 396593e

"qt": "Qt5Agg",

ipython/IPython/core/pylabtools.py

Lines 301 to 322 in 396593e

    
           def find_gui_and_backend(gui=None, gui_select=None): 
        
               """Given a gui string return the gui and mpl backend. 
        
               Parameters 
        
               ---------- 
        
               gui : str 
        
                   Can be one of ('tk','gtk','wx','qt','qt4','inline','agg'). 
        
               gui_select : str 
        
                   Can be one of ('tk','gtk','wx','qt','qt4','inline'). 
        
                   This is any gui already selected by the shell. 
        
               Returns 
        
               ------- 
        
               A tuple of (gui, backend) where backend is one of ('TkAgg','GTKAgg', 
        
               'WXAgg','Qt4Agg','module://matplotlib_inline.backend_inline','agg'). 
        
               """ 
        
               import matplotlib 
        
               if gui and gui != 'auto': 
        
                   # select backend based on requested gui 
        
                   backend = backends[gui]

This isn't documented and has caused me a lot of confusion and trouble (and will cause further trouble in the future as usage of Qt6 increases over Qt5):

I found out about this after spending months trying to work out why %matplotlib qt doesn't work when I have Qt6 bindings installed: matplotlib/matplotlib#25673 (comment)

I suggest we either explicitly document this in the IPython line magic docs, or we change this default behavior.

Relevant but not duplicate:

Support for PySide6 via %gui #13859

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	def find_gui_and_backend(gui=None, gui_select=None):
	"""Given a gui string return the gui and mpl backend.

	Parameters
	----------
	gui : str
	Can be one of ('tk','gtk','wx','qt','qt4','inline','agg').
	gui_select : str
	Can be one of ('tk','gtk','wx','qt','qt4','inline').
	This is any gui already selected by the shell.

	Returns
	-------
	A tuple of (gui, backend) where backend is one of ('TkAgg','GTKAgg',
	'WXAgg','Qt4Agg','module://matplotlib_inline.backend_inline','agg').
	"""

	import matplotlib

	if gui and gui != 'auto':
	# select backend based on requested gui
	backend = backends[gui]

Uh oh!

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