swiften, pypy: Mark as broken

swiften is broken since 2018-10-18:
x86_64-linux:  https://hydra.nixos.org/build/97180305
aarch64-linux: https://hydra.nixos.org/build/97193043

pypy is broken since 2019-05-29:
x86_64-linux (pypy):  https://hydra.nixos.org/build/97214777
x86_64-linux (pypy3): https://hydra.nixos.org/build/97221287

The builds for swiften and reposurgeon (depends on pypy) regularly fail
during rebuilds for SCons updates (e.g. [0]).

[0]: https://github.com/NixOS/nixpkgs/pull/65392