PyVTKObject: own the classname memory

In Python3, we take the actual object and construct a new bytes() object
from it. We then take a pointer to its data and decref the bytes object.
If the garbage collector runs between the decref and the use of the
classname variable further down, it is a use-after-free.

Wrapping/PythonCore/PyVTKObject.cxx

@@ -487,14 +487,14 @@ PyObject *PyVTKObject_FromPointer(
 {
   // This will be set if we create a new C++ object
   bool created = false;
-  const char *classname = vtkPythonUtil::StripModule(pytype->tp_name);
+  std::string classname = vtkPythonUtil::StripModule(pytype->tp_name);
   PyVTKClass *cls = 0;
 
   if (ptr)
     {
     // If constructing from an existing C++ object, use its actual class
     classname = ptr->GetClassName();
-    cls = vtkPythonUtil::FindClass(classname);
+    cls = vtkPythonUtil::FindClass(classname.c_str());
     }
 
   if (cls == 0)
@@ -511,14 +511,15 @@ PyObject *PyVTKObject_FromPointer(
         s = tmp;
         }
 #endif
-      classname = PyBytes_AsString(s);
-      if (classname == 0)
+      const char *vtkname_classname = PyBytes_AsString(s);
+      if (vtkname_classname == 0)
         {
         Py_DECREF(s);
         return NULL;
         }
+      classname = vtkname_classname;
       }
-    cls = vtkPythonUtil::FindClass(classname);
+    cls = vtkPythonUtil::FindClass(classname.c_str());
     if (cls == 0)
       {
       PyErr_Format(PyExc_ValueError,
@@ -550,7 +551,7 @@ PyObject *PyVTKObject_FromPointer(
 
       // Check the type of the newly-created object
       const char *newclassname = ptr->GetClassName();
-      if (strcmp(newclassname, classname) != 0)
+      if (newclassname == classname)
         {
         PyVTKClass *newclass = vtkPythonUtil::FindClass(newclassname);
         if (newclass)