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 *.

Python/modsupport.c

@@ -26,7 +26,7 @@ static char api_version_warning[] =
  This Python has API version %d, module %.100s has version %d.";
 
 PyObject *
-Py_InitModule4(char *name, PyMethodDef *methods, char *doc,
+Py_InitModule4(const char *name, PyMethodDef *methods, const char *doc,
 	       PyObject *passthrough, int module_api_version)
 {
 	PyObject *m, *d, *v, *n;
@@ -99,7 +99,7 @@ Py_InitModule4(char *name, PyMethodDef *methods, char *doc,
 /* Helper for mkvalue() to scan the length of a format */
 
 static int
-countformat(char *format, int endchar)
+countformat(const char *format, int endchar)
 {
 	int count = 0;
 	int level = 0;
@@ -142,14 +142,14 @@ countformat(char *format, int endchar)
 /* Generic function to create a value -- the inverse of getargs() */
 /* After an original idea and first implementation by Steven Miale */
 
-static PyObject *do_mktuple(char**, va_list *, int, int);
-static PyObject *do_mklist(char**, va_list *, int, int);
-static PyObject *do_mkdict(char**, va_list *, int, int);
-static PyObject *do_mkvalue(char**, va_list *);
+static PyObject *do_mktuple(const char**, va_list *, int, int);
+static PyObject *do_mklist(const char**, va_list *, int, int);
+static PyObject *do_mkdict(const char**, va_list *, int, int);
+static PyObject *do_mkvalue(const char**, va_list *);
 
 
 static PyObject *
-do_mkdict(char **p_format, va_list *p_va, int endchar, int n)
+do_mkdict(const char **p_format, va_list *p_va, int endchar, int n)
 {
 	PyObject *d;
 	int i;
@@ -195,7 +195,7 @@ do_mkdict(char **p_format, va_list *p_va, int endchar, int n)
 }
 
 static PyObject *
-do_mklist(char **p_format, va_list *p_va, int endchar, int n)
+do_mklist(const char **p_format, va_list *p_va, int endchar, int n)
 {
 	PyObject *v;
 	int i;
@@ -242,7 +242,7 @@ _ustrlen(Py_UNICODE *u)
 #endif
 
 static PyObject *
-do_mktuple(char **p_format, va_list *p_va, int endchar, int n)
+do_mktuple(const char **p_format, va_list *p_va, int endchar, int n)
 {
 	PyObject *v;
 	int i;
@@ -278,7 +278,7 @@ do_mktuple(char **p_format, va_list *p_va, int endchar, int n)
 }
 
 static PyObject *
-do_mkvalue(char **p_format, va_list *p_va)
+do_mkvalue(const char **p_format, va_list *p_va)
 {
 	for (;;) {
 		switch (*(*p_format)++) {
@@ -454,7 +454,7 @@ do_mkvalue(char **p_format, va_list *p_va)
 
 
 PyObject *
-Py_BuildValue(char *format, ...)
+Py_BuildValue(const char *format, ...)
 {
 	va_list va;
 	PyObject* retval;
@@ -465,9 +465,9 @@ Py_BuildValue(char *format, ...)
 }
 
 PyObject *
-Py_VaBuildValue(char *format, va_list va)
+Py_VaBuildValue(const char *format, va_list va)
 {
-	char *f = format;
+	const char *f = format;
 	int n = countformat(f, '\0');
 	va_list lva;
 
@@ -494,7 +494,7 @@ Py_VaBuildValue(char *format, va_list va)
 
 
 PyObject *
-PyEval_CallFunction(PyObject *obj, char *format, ...)
+PyEval_CallFunction(PyObject *obj, const char *format, ...)
 {
 	va_list vargs;
 	PyObject *args;
@@ -516,7 +516,7 @@ PyEval_CallFunction(PyObject *obj, char *format, ...)
 
 
 PyObject *
-PyEval_CallMethod(PyObject *obj, char *methodname, char *format, ...)
+PyEval_CallMethod(PyObject *obj, const char *methodname, const char *format, ...)
 {
 	va_list vargs;
 	PyObject *meth;
@@ -545,7 +545,7 @@ PyEval_CallMethod(PyObject *obj, char *methodname, char *format, ...)
 }
 
 int
-PyModule_AddObject(PyObject *m, char *name, PyObject *o)
+PyModule_AddObject(PyObject *m, const char *name, PyObject *o)
 {
 	PyObject *dict;
 	if (!PyModule_Check(m)) {
@@ -574,13 +574,13 @@ PyModule_AddObject(PyObject *m, char *name, PyObject *o)
 }
 
 int 
-PyModule_AddIntConstant(PyObject *m, char *name, long value)
+PyModule_AddIntConstant(PyObject *m, const char *name, long value)
 {
 	return PyModule_AddObject(m, name, PyInt_FromLong(value));
 }
 
 int 
-PyModule_AddStringConstant(PyObject *m, char *name, char *value)
+PyModule_AddStringConstant(PyObject *m, const char *name, const char *value)
 {
 	return PyModule_AddObject(m, name, PyString_FromString(value));
 }