Add the const qualifier to "char *" variables that refer to literal strings. (#4370)

diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index 17ec182..3d63186 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -1300,7 +1300,7 @@
 {
     PyObject *v, *w, *errtype, *errtext;
     PyObject *msg_obj = NULL;
-    char *msg = NULL;
+    const char *msg = NULL;
     int offset = err->offset;
 
     errtype = PyExc_SyntaxError;