buildbot: fix dependencies

buildbot was missing msgpack as a propagated build input, but somehow it was
available at build time so the failure didn't show up until runtime. I also
removed unidiff, which was an optional dependency that was accidentally made
required in 3.1.0 and fixed in 3.2.0.

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

@@ -1,8 +1,9 @@
 { stdenv, lib, buildPythonPackage, fetchPypi, makeWrapper, isPy3k
-, python, twisted, jinja2, zope_interface, sqlalchemy, alembic, python-dateutil
-, txaio, autobahn, pyjwt, pyyaml, unidiff, treq, txrequests, pypugjs, boto3
-, moto, mock, lz4, setuptoolsTrial, isort, pylint, flake8, buildbot-worker
-, buildbot-pkg, buildbot-plugins, parameterized, git, openssh, glibcLocales
+, python, twisted, jinja2, msgpack, zope_interface, sqlalchemy, alembic
+, python-dateutil, txaio, autobahn, pyjwt, pyyaml, treq, txrequests, pypugjs
+, boto3, moto, mock, lz4, setuptoolsTrial, isort, pylint, flake8
+, buildbot-worker, buildbot-pkg, buildbot-plugins, parameterized, git, openssh
+, glibcLocales
 , nixosTests
 }:
 
@@ -42,6 +43,7 @@ let
       # core
       twisted
       jinja2
+      msgpack
       zope_interface
       sqlalchemy
       alembic
@@ -50,7 +52,6 @@ let
       autobahn
       pyjwt
       pyyaml
-      unidiff
     ]
       # tls
       ++ twisted.extras-require.tls;