Increase upper bound on async

Fixes commercialhaskell/stackage#3268

turtle.cabal

@@ -51,7 +51,7 @@ Library
     Build-Depends:
         base                 >= 4.6     && < 5   ,
         ansi-wl-pprint       >= 0.6     && < 0.7 ,
-        async                >= 2.0.0.0 && < 2.2 ,
+        async                >= 2.0.0.0 && < 2.3 ,
         bytestring           >= 0.9.1.8 && < 0.11,
         clock                >= 0.4.1.2 && < 0.8 ,
         containers           >= 0.5.0.0 && < 0.6 ,