Allow building with `transformers-0.6.*`

[RyanGlScott]

If I attempt to relax the upper version bound of transformers-0.6.* for contravariant-1.6 on Hackage, it will fail with:

[1 of 3] Compiling Data.Functor.Contravariant.Divisible ( src/Data/Functor/Contravariant/Divisible.hs, dist/build/Data/Functor/Contravariant/Divisible.o, dist/build/Data/Functor/Contravariant/Divisible.dyn_o )

src/Data/Functor/Contravariant/Divisible.hs:37:1: error:
    Could not load module ‘Control.Monad.Trans.Error’
    It is a member of the hidden package ‘transformers-0.5.6.2’.
    Perhaps you need to add ‘transformers’ to the build-depends in your .cabal file.
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
37 | import Control.Monad.Trans.Error
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/Data/Functor/Contravariant/Divisible.hs:40:1: error:
    Could not load module ‘Control.Monad.Trans.List’
    It is a member of the hidden package ‘transformers-0.5.6.2’.
    Perhaps you need to add ‘transformers’ to the build-depends in your .cabal file.
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
40 | import Control.Monad.Trans.List
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[RyanGlScott]

I've pushed:

• 575c60c to the 1.5 branch, which I've released as contravariant-1.5.4, and
• 9f89427 to the main branch, which will appear in the next major release (contravariant-1.6).