mtl-2.3.1 compatibility

[ysangkok]

The upcoming mtl-2.3.1 release will not re-export MonadIO(liftIO) from
unrelated modules. So let's add an additional (backwards compatible) import.
This keeps hedis working, even with the latest mtl release (which is currently not compatible).

Tested using the following cabal.project:

packages: .

constraints: mtl==2.3.1

[ysangkok]

@aravindgopall This is ready for review :)

[ysangkok]

@aravindgopall Given that this was a source code change, users of mtl-2.3 (e.g. GHC-9.6+ users) can't use hedis unless they use extra-deps (Stack) or source-repository-package (Cabal). If this change was released on Hackage, that wouldn't be necessary. Are you planning to make a release? I don't think I see your name on the list of maintainers. Would you like to get added to that list?