Declaring a (Q)Property as "constant" does not work

Since PySide2 5.12.0, declaring a Property as constant using constructor parameter "constant=True" does not work anymore.

Attached is a minimal test case that shows that the corresponding QMetaProperty returns False when calling "isConstant()" (worked as expected before 5.12.0).

This is noticeable when using constant properties from QML, which raises the "depends on non-NOTIFYable properties:" warning.