Qt6: fix possible build failure

* add CMake definitions 'QT_NO_KEYWORDS' to avoid the Qt definition of the 'slots' macro that causes a conflict when including Python headers
* drop QT3_SUPPORT define

cMake/FreeCAD_Helpers/SetupQt.cmake

@@ -38,6 +38,8 @@ set(CMAKE_AUTOMOC TRUE)
 set(CMAKE_AUTOUIC TRUE)
 set(QtCore_MOC_EXECUTABLE ${Qt${FREECAD_QT_MAJOR_VERSION}Core_MOC_EXECUTABLE})
 
+add_definitions(-DQT_NO_KEYWORDS)
+
 message(STATUS "Set up to compile with Qt ${Qt${FREECAD_QT_MAJOR_VERSION}Core_VERSION}")
 
 # In Qt 5.15 they added more generic names for these functions: "backport" those new names