OCaml 4.13 support

[Fourchaux]

It would be great to have OCaml 4.13 support.

TY

With OCaml 4.13.1+flambda I have:

$ opam install DrawGrammar
[ERROR] Package conflict!
  * No agreement on the version of ocaml:
    - (invariant) → ocaml-variants = 4.13.1+options → ocaml = 4.13.1
    - DrawGrammar → ocaml < 4.06.0
    You can temporarily relax the switch invariant with `--update-invariant'

No solution found, exiting