[Python.NET] Wrap in interface (#1240)

[Mohamed Koubaa]

Hello,

This PR<https://github.com/pythonnet/pythonnet/pull/1240> changed behavior of 
pythonnet to use stricter typing when a method returns an interface.  I'd like 
a way to opt-out of this behavior, and I'll share a representative use-case 
below.  The tl;dr is that python is a dynamic language and I expect the C# 
objects which pythonnet wraps to behave more like "dynamic" and less like the 
static types on the interface.

**** C# code ****
namespace Example1
{
    public enum ShapeType
    {
        Square,
        Circle
    }
    public interface IShape
    {
        void Draw();

        double Area { get; }

        ShapeType Type { get; }
    }

    public class Square : IShape
    {
        public double Length { get; }
        public Square(double length)
        {
            Length = length;
        }

        public void Draw() {}
        public double Area { get { return Length * Length; } }
        public ShapeType Type { get { return ShapeType.Square; } }
    }

    public class Circle : IShape
    {
        public double Radius { get; }
        public Circle(double radius)
        {
            Radius = radius;
        }

        public void Draw() {}
        public double Area { get { return Math.PI * Radius * Radius; } }
        public ShapeType Type { get { return ShapeType.Circle; } }
    }

    public class ShapeDataModel
    {
        private Dictionary<int, IShape> _shapes = new Dictionary<int, IShape>();
        private int _nextId = 1;

        public int AddShape(IShape shape)
        {
            int id = _nextId;
            _shapes[id] = shape;
            _nextId++;
            return id;
        }

        public IShape GetShape(int id)
        {
            return _shapes[id];
        }
    }
}


**** Python code ****
>>> import clr
>>> clr.AddReference("Example1")
>>> import Example1
>>> sq1 = Example1.Square(2.)
>>> ci1 = Example1.Circle(1.3)
>>> sq2 = Example1.Square(2.5)
>>> m = Example1.ShapeDataModel()
>>> m.AddShape(sq1)
1
>>> m.AddShape(sq2)
2
>>> m.AddShape(ci1)
3
>>> m.GetShape(2)
<Example1.IShape object at 0x000002A3448A09A0>
>>> m.GetShape(2).Area
6.25
>>> m.GetShape(2).Length
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'IShape' object has no attribute 'Length'

**** summary ****
This factory and/or datamodel pattern is very common in my codebase and 
probably also lots of object oriented systems.  The mentioned PR breaks this 
common design pattern in C# objects wrapped by pythonnet, and I would like a 
way to opt-out of it.  Maybe an attribute on the method which returns an 
interface can be used, or maybe something global to switch this behavior.  I 
think perhaps the original bug might also have been fixed in a different way.

Thanks!
Mohamed

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