ghc-8.4.4: use sphinx 1.7.x to build the documentation

ghc-8.4.4 requires sphinx < 1.8, otherwise build fails on haddock with:

    Extension error: The 'ghc-flag' directive is already registered to domain std

Also fixed evaluation errors in configurations of ghc-8.2.x and ghc-8.4.x.

Closes https://github.com/NixOS/nixpkgs/pull/55703.