python/twisted: Fix fetching source

In ced21f5e

, 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