[3.8] closes bpo-39630: Update pointers to string literals to be const char *. (GH-18511)

(cherry picked from commit 7386a70746cf9aaf2d95db75d9201fb124f085df)

Co-authored-by: Andy Lester <andy@petdance.com>
diff --git a/Objects/genobject.c b/Objects/genobject.c
index 2c06bdc..ce7dd48 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -11,10 +11,10 @@
 static PyObject *async_gen_asend_new(PyAsyncGenObject *, PyObject *);
 static PyObject *async_gen_athrow_new(PyAsyncGenObject *, PyObject *);
 
-static char *NON_INIT_CORO_MSG = "can't send non-None value to a "
+static const char *NON_INIT_CORO_MSG = "can't send non-None value to a "
                                  "just-started coroutine";
 
-static char *ASYNC_GEN_IGNORED_EXIT_MSG =
+static const char *ASYNC_GEN_IGNORED_EXIT_MSG =
                                  "async generator ignored GeneratorExit";
 
 static inline int