Comment 1 for bug 1697613

I checked if there are missing bindep entries other than libffi-dev, but it looks like libffi-dev is the only one.

I checked it in the following steps.

- Prepare Ubuntu 16.04 with minimum installation
- Create a new virtualenv
- pip install bindep
- sudo apt-get install `bindep -b -r bindep.txt test'
- sudo apt-get install libffi-dev
- pip install --no-binary :all: -r requirements.txt -r test-requirements.txt
(we can avoid using wheel packages by --no-binary :all: option)