I discussed this off-list with GvR.  He was primarily concerned with
fixing the passing-around-a-vtable C API usage of CObject, but he wanted
to preserve as much backwards compatibility as possible.  In the end, he
suggested I create a new API and leave CObject unchanged.  I've done
that, incorporating many of GvR's suggestions, though the blame for the
proposed new API is ultimately mine.

The new object is called a "Capsule".  (I *had* wanted to call it
"Wrapper", but there's already a PyWrapper_New in descrobject.h.) 
Highlights of the new API:
* PyCapsule_New() replaces PyCObject_FromVoidPtr.
  * It takes a void * pointer, a const char *name, and a destructor.
  * The pointer must not be NULL.
  * The name may be NULL; if it is not NULL, it must be a valid C string
which outlives the capsule.
  * The destructor takes a PyObject *, not a void *.
* PyCapsule_GetPointer() replaces PyCObject_AsVoidPtr.
  * It takes a PyObject * and a const char *name.
  * The name must compare to the name inside the object; either they're
both NULL or they strcmp to be the same.
* PyCapsule_Import() replaces PyCObject_Import.
  * It takes three arguments: const char *module_name, const char
*attribute_name, int no_block.
  * It ensures that the "name" of the Capsule is "modulename.attributename".
  * If no_block is true, it uses PyModule_ImportModuleNoBlock.
* The PyCapsule structure is private.  There are accessors for all
fields: pointer, name, destructor, and "context"
  * The "context" is a second "void *" you can set / get.

I've attached a patch to implement this; it was written against svn
r71304.  The patch isn't ready to be applied--there is no documentation
for the new API beyond the header file.

GvR and I disagree on one point: he thinks that we should leave CObject
in forever, undeprecated.  I think we should deprecate it now and remove
it... whenever we'd do that.  The new API does everything the old one
does, and more, and it's cleaner and safer.