Changed applicable use of ``char *`` declarations that are passed into

PyArg_ParseTuple() to ``const char *`` to match the recommendation made in
section 1.3 and to support better coding habits.

Section 1.8 ("Keyword Parameters for Extension Functions") and it's coding
example were not touched since it is stems from an accredited source and thus
did not want to step on anyone's toes.

Doc/ext/extending.tex

@@ -70,7 +70,7 @@ is evaluated (we'll see shortly how it ends up being called):
 static PyObject *
 spam_system(PyObject *self, PyObject *args)
 {
-    char *command;
+    const char *command;
     int sts;
 
     if (!PyArg_ParseTuple(args, "s", &command))
@@ -634,7 +634,7 @@ Some example calls:
     int ok;
     int i, j;
     long k, l;
-    char *s;
+    const char *s;
     int size;
 
     ok = PyArg_ParseTuple(args, ""); /* No arguments */
@@ -659,8 +659,8 @@ Some example calls:
 
 \begin{verbatim}
     {
-        char *file;
-        char *mode = "r";
+        const char *file;
+        const char *mode = "r";
         int bufsize = 0;
         ok = PyArg_ParseTuple(args, "s|si", &file, &mode, &bufsize);
         /* A string, and optionally another string and an integer */
@@ -1228,7 +1228,7 @@ declared \keyword{static} like everything else:
 
 \begin{verbatim}
 static int
-PySpam_System(char *command)
+PySpam_System(const char *command)
 {
     return system(command);
 }
@@ -1240,7 +1240,7 @@ The function \cfunction{spam_system()} is modified in a trivial way:
 static PyObject *
 spam_system(PyObject *self, PyObject *args)
 {
-    char *command;
+    const char *command;
     int sts;
 
     if (!PyArg_ParseTuple(args, "s", &command))