Fix: add missing dependency constraint on pyml in the opam file

Reported by kit-ty-kate: https://github.com/coccinelle/coccinelle/pull/198/commits/3ae8708b342394db1a2f1689774ef9496f0d8c86

With pyml version <20171117, build fails with the following error:

File "python/yes_pycocci.ml", line 77, characters 15-46:
 77 |       ~parents:[pycocci_get_class_type parent]
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Error: This expression has type 'a list
        but an expression was expected of type Py.Object.t =
        Pytypes.pyobject

Indeed, the signature of Py.Class.init (the function which is being applied here) changed between 20170807 and 20171117. For reference, the interface was changed by the commit: https://github.com/thierry-martinez/pyml/commit/2fedb0bc2d27de89668d21760dbc1f90d22f7112