Add setupcfg2nix and supporting infrastructure

pkgs/build-support/build-setupcfg/default.nix

@@ -0,0 +1,23 @@
+# Build a python package from info made available by setupcfg2nix.
+#
+# * src: The source of the package.
+# * info: The package information generated by setupcfg2nix.
+# * meta: Standard nixpkgs metadata.
+# * application: Whether this package is a python library or an
+#   application which happens to be written in python.
+pythonPackages: { src, info, meta ? {}, application ? false }: let
+  build = if application
+    then pythonPackages.buildPythonApplication
+  else pythonPackages.buildPythonPackage;
+in build {
+  inherit (info) pname version;
+
+  inherit src meta;
+
+  nativeBuildInputs = map (p: pythonPackages.${p}) (
+    (info.setup_requires or []) ++
+    (info.tests_require or []));
+
+  propagatedBuildInputs = map (p: pythonPackages.${p})
+    (info.install_requires or []);
+}