bpy.props.EnumProperty return type should be type[set[str]] when the 'ENUM_FLAG' option is enabled

[Mysteryem]

Currently bpy.props.EnumProperty is modified to have the return type-hint type[str], but it should also have type[set[str]] as a possible return. I'm not sure if an @overload can be added for the function since the options parameter is a set.

PyCharm seems to have some issues if these functions return Unions of types or types of Unions, but it might have been limited to Generic types, will have to try it and see. Might have to try a hacky 'amalgamation' class that extends both str and set[str] and setting the return type-hint to type[HackyAmalgamation]