.travis.yml: Don't test sdist install on cabal 3.[0-3]

Since it doesn't work. It should be fixed in 3.4, see: haskell/cabal#6393

Author: Deewiant

.travis.yml

@@ -45,7 +45,13 @@ script:
  SRC_TGZ=$(cabal info . | awk '{print $2 ".tar.gz";exit}')
  cd dist-newstyle/sdist || cd dist
  if [[ -f "$SRC_TGZ" ]]; then
- cabal install --force-reinstalls "$SRC_TGZ"
+ if cabal --version | grep -q 'version 3.[0-3]'; then
+ # https://github.com/haskell/cabal/issues/6393
+ echo >&2 "cabal v2-install broke tarball installs, fixed in 3.4"
+ echo >&2 "Will not test sdist installability"
+ else
+ cabal install --force-reinstalls "$SRC_TGZ"
+ fi
  else
  echo >&2 "$SRC_TGZ not found"
  exit 1