Arvados

https://github.com/curoverse/arvados/blob/master/sdk/python/setup.py
'ruamel.yaml >=0.13.7'

https://github.com/curoverse/arvados/blob/master/sdk/cwl/setup.py
'ruamel.yaml==0.13.7'

A typical install:
pip install arvados-python-client arvados-cwl-runner

The listed pip call installs the latest ruamel version and conflicts with arvados-cwl-runner.

You can reproduce as follows:
docker run -it ubuntu:16.04
apt-get update
apt-get install -y wget bzip2 curl libcurl4-openssl-dev gcc libssl-dev nano
wget https://repo.continuum.io/miniconda/Miniconda2-latest-Linux-x86_64.sh -O miniconda.sh;
bash miniconda.sh -b -p $HOME/miniconda
export PATH="$HOME/miniconda/bin:$PATH"
conda update -y setuptools
conda install -y pyopenssl
pip install -U arvados-python-client arvados-cwl-runner