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 *.

Modules/_tkinter.c

@@ -2393,7 +2393,7 @@ Tktt_Repr(PyObject *self)
 }
 
 static PyObject *
-Tktt_GetAttr(PyObject *self, char *name)
+Tktt_GetAttr(PyObject *self, const char *name)
 {
 	return Py_FindMethod(Tktt_methods, self, name);
 }
@@ -2723,7 +2723,7 @@ Tkapp_Dealloc(PyObject *self)
 }
 
 static PyObject *
-Tkapp_GetAttr(PyObject *self, char *name)
+Tkapp_GetAttr(PyObject *self, const char *name)
 {
 	return Py_FindMethod(Tkapp_methods, self, name);
 }