Cabal revisions of mtl-2.3.1

Hackage metadata revisions edit the .cabal file after upload; each diff below is one revision.

 cabal-version: 3.0 name:          mtl version:       2.3.1+x-revision: 1 license:       BSD-3-Clause license-file:  LICENSE author:        Andy Gill     Control.Monad.Select    build-depends:-    , base >=4.12 && < 5+    , base >=4.12 && <4.15 || >=4.16 && < 5     , transformers >= 0.5.6 && <0.7    ghc-options: