python3Packages.packaging: change build system from flit to setuptools

With 20.8 upstream reverted buildsystem from flit back to setuptools as
this would make it easier for some distro's.

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

@@ -5,7 +5,7 @@
 , six
 , pytestCheckHook
 , pretend
-, flit-core
+, setuptools
 }:
 
 buildPythonPackage rec {
@@ -19,7 +19,7 @@ buildPythonPackage rec {
   };
 
   nativeBuildInputs = [
-    flit-core
+    setuptools
   ];
 
   propagatedBuildInputs = [ pyparsing six ];