Adds missing test dependency to certifi.

dev_requirements.txt

@@ -19,6 +19,7 @@ six
 
 # those are the additional packages required to run the tests
 pyfakefs
+certifi
 ddt
 mock
 pytest # optional (python setup.py test works without it), but possible nonetheless

setup.py

@@ -60,7 +60,7 @@ setup(
     ],
 
     test_suite='tests',
-    tests_require=['pyfakefs>=3.4', 'mock', 'ddt'],
+    tests_require=['pyfakefs>=3.4', 'mock', 'ddt', 'certifi'],
 
     # in order to avoid 'zipimport.ZipImportError: bad local file header'
     zip_safe=False,