python/twisted: Fix fetching source

In ced21f5e1a, fetchurl has changed into
fetchPypi, but the upstream tarball is actually a tar.bz2 archive but by
default fetchPypi uses "tar.gz" as its extension.

The sha256 however is still the same.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @FRidh

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

@@ -17,6 +17,7 @@ buildPythonPackage rec {
 
   src = fetchPypi {
     inherit pname version;
+    extension = "tar.bz2";
     sha256 = "a4cc164a781859c74de47f17f0e85f4bce8a3321a9d0892c015c8f80c4158ad9";
   };