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 *.

Objects/listobject.c

@@ -1983,7 +1983,7 @@ listsort(PyListObject *self, PyObject *args, PyObject *kwds)
 	PyObject *keyfunc = NULL;
 	int i;
 	PyObject *key, *value, *kvpair;
-	static char *kwlist[] = {"cmp", "key", "reverse", 0};
+	static const char *kwlist[] = {"cmp", "key", "reverse", 0};
 
 	assert(self != NULL);
 	assert (PyList_Check(self));
@@ -2357,7 +2357,7 @@ static int
 list_init(PyListObject *self, PyObject *args, PyObject *kw)
 {
 	PyObject *arg = NULL;
-	static char *kwlist[] = {"sequence", 0};
+	static const char *kwlist[] = {"sequence", 0};
 
 	if (!PyArg_ParseTupleAndKeywords(args, kw, "|O:list", kwlist, &arg))
 		return -1;