Attempt to fix python 2 build

Author: Sylvain MARIE

.travis.yml

@@ -26,6 +26,7 @@ install:
   - pip list
   - pip install setuptools_scm  # apparently python 2 does not support that it is installed simultaneously with pytest-cases
   - pip install six pypandoc        # again just for python 2 build because it uses an old pytest-cases that needs it
+  - pip install pytest_cases==1.10.1  # again just for python 2 !
   - python ci_tools/py_install.py pip ci_tools/requirements-pip.txt
   # this does not work anymore on python 2 so lets only do it when needed
   - if [ "${TRAVIS_PYTHON_VERSION}" = "3.5" ]; then pip install mkdocs-material mkdocs; fi;