pythonPackages.BTrees: fix evaluation; builds fine

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

···
-{ stdenv, buildPythonPackage, persistent, zope_interface, transaction }:
+{ stdenv, fetchPypi, buildPythonPackage, persistent, zope_interface, transaction }:
 
 buildPythonPackage rec {
   pname = "BTrees";