Use older version of http2 to avoid breakage with -haddock on 8.10

Author: wz1000

cabal.project

@@ -66,7 +66,9 @@ constraints:
   ghc-check -ghc-check-use-package-abis,
   ghc-lib-parser-ex -auto,
   stylish-haskell +ghc-lib,
-  fourmolu -fixity-th
+  fourmolu -fixity-th,
+  -- http2 doesn't build with -haddock on ghc-8.10
+  http2 < 4.0.0
 
 -- This is benign and won't affect our ability to release to Hackage,
 -- because we only depend on `ekg-json` when a non-default flag