2.3.1

CHANGES:

Added

• Add os_type label to enable/disable based on Sys.os_type (#433,
  @polytypic)

• Make MDX compatible with OCaml 5.1 (#435, @polytypic and @kit-ty-kate)

Changed

• Vendored the odoc-parser library, removing the need to have it
  as a dependency. (#430, @jonludlam)