[python] Fix inheriting from multiple director classes

Fix mishandling of a Python class inheriting from multiple SWIG-wrapped
director classes.

Fixes #422
Fixes https://sourceforge.net/p/swig/bugs/1379/

Examples/test-suite/director_multiple_inheritance.i

@@ -0,0 +1,71 @@
+//%module(ruby_minherit="1") multiple_inheritance
+%module(directors="1") director_multiple_inheritance
+
+%feature("director") A;
+%feature("director") B;
+%feature("director") C;
+%feature("director") D;
+
+%inline %{
+
+class A {
+public:
+    virtual ~A() { }
+    virtual int testA();
+};
+
+class B: virtual public A {
+public:
+    virtual ~B() { }
+    virtual int testB();
+};
+
+class C: virtual public A {
+public:
+    virtual ~C() { }
+    virtual int testC();
+};
+
+class D: virtual public A {
+public:
+    virtual ~D() { }
+    virtual int testD();
+};
+
+class E {
+public:
+    virtual ~E() { }
+    virtual int testE(B*);
+};
+
+class F {
+public:
+    virtual ~F() { }
+    virtual int testF(C*);
+};
+
+class T {
+public:
+    virtual ~T() { }
+    virtual int testT(D*);
+};
+
+%}
+
+%{
+
+int A::testA() { return 1; }
+
+int B::testB() { return 2; }
+
+int C::testC() { return 3; }
+
+int D::testD() { return 4; }
+
+int E::testE(B*) { return 5; }
+
+int F::testF(C*) { return 6; }
+
+int T::testT(D*) { return 20; }
+
+%}