Constrain sexplib version
Constrain sexplib version

sexplib is a development dependency of odoc. We need at least version
113.33.00, for consistent escaping of characters with numeric values
above 127. See

  https://github.com/janestreet/sexplib/blob/57d2a5a03bfcfe5a81a003b1e112283cf6a4ccbd/CHANGES.md#1133300

ocaml-doc/odoc,ocaml-doc/odoc