bpo-39943: Add the const qualifier to pointers on non-mutable PyBytes data. (GH-19472)

diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 12dba38..92d6faa 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -2640,7 +2640,7 @@
     Py_ssize_t chars_to_skip, chars_decoded;
     Py_ssize_t skip_bytes, skip_back;
     PyObject *saved_state = NULL;
-    char *input, *input_end;
+    const char *input, *input_end;
     Py_ssize_t dec_buffer_len;
     int dec_flags;