pythonPackages.devpi-common: fix meta evaluation

The tarball job now seems OK again :-)

pkgs/development/python-modules/devpi-common/default.nix

···
-{ pythonPackages }:
+{ stdenv, pythonPackages }:
 
 with pythonPackages;buildPythonPackage rec {
   pname = "devpi-common";
···
     py.test
   '';
 
-  meta = {
+  meta = with stdenv.lib; {
     homepage = https://github.com/devpi/devpi;
     description = "Utilities jointly used by devpi-server and devpi-client";
     license = licenses.mit;