[CO] Investigating/using "qtpy" for the GUI

[BonneelP]

Hello all,

Hélène just found an interesting package to investigate: https://github.com/spyder-ide/qtpy. This package "provides an uniform layer to support PyQt5, PySide2, PyQt4 and PySide with a single codebase". It could help us solve the issue we have with swat-em for instance (cf #378).

Best regards, Pierre