Preemptively backport the relevant parts of r65420

Lib/test/test_str.py

@@ -364,6 +364,9 @@ class StrTest(
         self.assertRaises(ValueError, format, "", "-")
         self.assertRaises(ValueError, "{0:=s}".format, '')
 
+    def test_buffer_is_readonly(self):
+        self.assertRaises(TypeError, sys.stdin.readinto, b"")
+
 
 def test_main():
     test_support.run_unittest(StrTest)

Objects/stringobject.c

@@ -1329,7 +1329,7 @@ static int
 string_buffer_getbuffer(PyStringObject *self, Py_buffer *view, int flags)
 {
 	return PyBuffer_FillInfo(view, (void *)self->ob_sval, Py_SIZE(self),
-				 0, flags);
+				 1, flags);
 }
 
 static PySequenceMethods string_as_sequence = {