html5validator: use callPackage instead of python3Packages.callPackage

pkgs/top-level/all-packages.nix

···
 
   inherit (gridlock) nyarr;
 
-  html5validator = python3Packages.callPackage ../applications/misc/html5validator { };
+  html5validator = callPackage ../applications/misc/html5validator { };
 
   inspec = callPackage ../tools/misc/inspec { };