requirements.txt: require msgpack 0.5.0
requirements.txt: require msgpack 0.5.0

msgpack was renamed from msgpack-python. msgpack-python became an empty
transition packages that depends on msgpack, but on upgrades the actual
code files are removed.

See: https://github.com/pypa/pip/issues/4961

terceiro/squad,terceiro/squad,terceiro/squad,terceiro/squad