mpi4py: Disable for Python3 and PyPy

Builds for Python3 and PyPy are failing at the moment [1-3]. Therefore, they
are disabled.

[1]: https://github.com/NixOS/nixpkgs/pull/3938#issuecomment-54750065
[2]: http://hydra.nixos.org/build/13993137/nixlog/1/raw
[3]: http://hydra.nixos.org/build/13873028/log/raw
Merge pull request #4001 from aherrmann/pr_mpi

mpi4py: Disable for Python3 and PyPy