Changing preferred Qt binding from pyqt5 to qt6

[t20100]

Qt5 has reaching the end of regular support mid-2023 and only receives updates under extended commercial support
silx issues occuring with Qt5 but not Qt6 starts to be reported (see #4135).

IMO, it's timely to consider changing the default Qt binding used by silx, this means:

• Changing the priorities of the different Qt bindings. Current priorities:
  

  silx/src/silx/gui/qt/_qt.py

  Line 67 in a7da263

  bindings = "PyQt5", "PySide6", "PyQt6"

• Changing the Qt binding installed with pip install silx[full]

Support of PyQt5 will remain, it will just no longer be selected by default.

The following question is: PySide6 or PyQt6 ?
I'm +1for PySide6.

[mretegan]

I don't see any downside to changing to PySide6, so +1.