Fixing setting this in Python when using __slots__

Don't attempt to use the class's __dict__ for setting 'this' when
a user has extended a class with:
    __slots__ = ['this'].
Was segfaulting. Now we fall back to a simple PyObject_SetAttr if the
usual approach to setting 'this' in __dict__ does not work.

Closes #1673 Closes #1674

Examples/test-suite/python_append.i

@@ -42,13 +42,31 @@ import os.path
 %}
 
 %inline %{
-
 class Test {
 public:
   static void static_func() {};
   void funk() {};
 };
+%}
 
+// Github issue #1674
+%extend ForSlots {
+  %pythoncode %{
+        __slots__ = ["this"]
+    %}
+}
+// While __slots__ does not contain 'ValidVariable' in the list, it is still possible
+// to set 'ValidVariable'. A little odd, but the whole attribute setting is bypassed
+// for setting C/C++ member variables.
+// Not sure how to test the equivalent for -builtin.
+%inline %{
+struct ForSlots {
+  int ValidVariable;
+  ForSlots() : ValidVariable(99) {}
+};
+%}
+
+%inline %{
 #ifdef SWIGPYTHON_BUILTIN
 bool is_python_builtin() { return true; }
 #else