bpo-21071: struct.Struct.format type is now str (#845)

diff --git a/Modules/_struct.c b/Modules/_struct.c
index 46e7b40..b4febf7 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1957,8 +1957,8 @@
 static PyObject *
 s_get_format(PyStructObject *self, void *unused)
 {
-    Py_INCREF(self->s_format);
-    return self->s_format;
+    return PyUnicode_FromStringAndSize(PyBytes_AS_STRING(self->s_format),
+                                       PyBytes_GET_SIZE(self->s_format));
 }
 
 static PyObject *