Add const to several API functions that take char *.

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 *.

Include/cStringIO.h

@@ -38,7 +38,7 @@ static struct PycStringIO_CAPI {
   int(*creadline)(PyObject *, char **);
 
   /* Write a string to an output object*/
-  int(*cwrite)(PyObject *, char *, int);
+  int(*cwrite)(PyObject *, const char *, int);
 
   /* Get the output object as a Python string (returns new reference). */
   PyObject *(*cgetvalue)(PyObject *);