pin ddt version to 1.3.1

because of https://github.com/datadriventests/ddt/issues/83

dev_requirements.txt

@@ -20,6 +20,8 @@ six
 # those are the additional packages required to run the tests
 pyfakefs
 certifi
-ddt
+# FIXME: remove strict version when https://github.com/datadriventests/ddt/issues/83 is fixed.
+#        (also remove in setup.py)
+ddt==1.3.1
 mock
 pytest # optional (python setup.py test works without it), but possible nonetheless

setup.py

@@ -61,7 +61,7 @@ setup(
     ],
 
     test_suite='tests',
-    tests_require=['pyfakefs>=3.4', 'mock', 'ddt', 'certifi'],
+    tests_require=['pyfakefs>=3.4', 'mock', 'ddt==1.3.1', 'certifi'],
 
     # in order to avoid 'zipimport.ZipImportError: bad local file header'
     zip_safe=False,