python3Packages.pygal: fix darwin sandbox build

pkgs/development/python-modules/pygal/default.nix

@@ -56,6 +56,10 @@ buildPythonPackage rec {
     export LANG=en_US.UTF-8
   '';
 
+  # Cairo tries to load system fonts by default.
+  # It's surfaced as a Cairo "out of memory" error in tests.
+  __impureHostDeps = [ "/System/Library/Fonts" ];
+
   postCheck = ''
     export LANG=${if stdenv.isDarwin then "en_US.UTF-8" else "C.UTF-8"}
   '';