python3Packages.pygrok: enable on darwin

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

···
     description = "A python implementation of jordansissel's grok regular expression library";
     homepage = "https://github.com/garyelephant/pygrok";
     license = licenses.mit;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }