Pin bisect_ppx to 2.5.0 to avoid circular dependency
Pin bisect_ppx to 2.5.0 to avoid circular dependency

This was failing in the github actions tests, as bisect_ppx 2.6.0 has
a test dependency on ocamlformat 0.16.0, which has a dependency on odoc.
I assume ocaml-ci doesn't install test dependencies for the main project
dependencies. Another problem due to the ocamldoc comment parser being
part of odoc.

Signed-off-by: Jon Ludlam <44f878afe53efc66b76772bd845eb65944ed8232@recoil.org>

ocaml-doc/odoc,ocaml-doc/odoc