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

Reported by kit-ty-kate:
3ae8708

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:
thierry-martinez/pyml@2fedb0b

coccinelle.opam

@@ -20,7 +20,7 @@ depends: [
   "ocamlfind"
   "pcre"
   "stdcompat"
-  "pyml"
+  "pyml" { >= "20171117" }
   "conf-pkg-config"
   "conf-python-3"
   "conf-python-3-dev"