CObject to PyCapsule.

backwards compatibility. Add Py_Refcnt, Py_Type, Py_Size, and
PyVarObject_HEAD_INIT.

In C++, it's an error to pass a string literal to a char* function
without a const_cast().  Rather than require every C++ extension
module to put a cast around string literals, fix the API to state the
const-ness.

I focused on parts of the API where people usually pass literals:
PyArg_ParseTuple() and friends, Py_BuildValue(), PyMethodDef, the type
slots, etc.  Predictably, there were a large set of functions that
needed to be fixed as a result of these changes.  The most pervasive
change was to make the keyword args list passed to
PyArg_ParseTupleAndKewords() to be a const char *kwlist[].

One cast was required as a result of the changes:  A type object
mallocs the memory for its tp_doc slot and later frees it.
PyTypeObject says that tp_doc is const char *; but if the type was
created by type_new(), we know it is safe to cast to char *.

Gernot Hillier added more detail to the internal API documentation.

Don't pollute the namespace when protecting against multiple header inclusion.
Prefix with Py_ and use standard naming convention Py_FILENAME_H.

xxxPyCObject_Import() seems to be a copy of PyCObject_Import().

permission from Paul Everitt).  Also removed a few other references to
Digital Creations and changed the remaining ones to Zope Corporation.