Fix FTBFS with Python 3.13.

This solves issue #32.

Signed-off-by: François Andriot <albator78@libertysurf.fr>
(cherry picked from commit d0ab12dc39)

sip/tqt/tqstring.sip

@@ -890,9 +890,16 @@ const char *PyTQt_tqt_encode(PyObject **s, TQApplication::Encoding encoding)
         es = PyBytes_AS_STRING(obj);
         Py_INCREF(obj);
     }
-    else if (PyObject_AsCharBuffer(obj, &es, &sz) >= 0)
+    else
     {
-        Py_INCREF(obj);
+        Py_buffer view;
+
+        if (PyObject_GetBuffer(obj, &view, PyBUF_SIMPLE) == 0) {
+            es = (const char*)view.buf;
+            sz = view.len;
+            PyBuffer_Release(&view);
+            Py_INCREF(obj);
+        }
     }
 
     if (es)