python3Packages.dropbox: Re-add setuptools to propagatedBuildInputs

It was removed in #170289, however dropbox/session.py still imports
pkg_resources, which is part of setuptools.

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

@@ -3,6 +3,7 @@
 , pythonOlder
 , fetchFromGitHub
 , requests
+, setuptools
 , six
 , stone
 , mock
@@ -28,6 +29,7 @@ buildPythonPackage rec {
 
   propagatedBuildInputs = [
     requests
+    setuptools
     six
     stone
   ];