package/homebrew: python-setuptools is a build dependency.

It worked before, but I guess everything else moved on to some other
buildsystem so it's not installed by default anymore? Or something?

package/homebrew/magnum-bindings.rb

@@ -8,6 +8,7 @@ class MagnumBindings < Formula
 
   depends_on "cmake"
   depends_on "python"
+  depends_on "python-setuptools" => :build
   depends_on "magnum"
   depends_on "pybind11" => :build