Natron 2.5.0 alpha 2

Sames changes as v2.4.3-rc1, plus:

• Use Python 3 rather than Python 2 (which is not maintained anymore). This will require very few changes to the Python code (mainly print statements), and most code can be easily adapted to be 2-3 compatible. #686
• Use QtPy==1.11.2 rather than PySide. QtPy is an abstraction layer for PyQt5/PyQt4/PySide2/PySide (newer versions support PyQt6/PySide6). QtPy uses the Qt5 layout (where the QtGui module has been split into QtGui and QtWidgets). Using QtPy guarantees a higher level of forward-compatibility with future versions of Natron, which may use Qt5 or Qt6. PySide can still be imported explicitly, if required, but the resulting code will not be forward-compatible. #687