Declare implicit dependency on Six 1.9 or higher

.travis.yml

@@ -16,6 +16,7 @@ cache:
 env:
   - USE_OPTIONAL=true
   - USE_OPTIONAL=false
+  - SIX_VERSION=1.9 USE_OPTIONAL=true
 
 install:
   - bash requirements-install.sh

requirements-install.sh

@@ -11,6 +11,10 @@ if [[ $USE_OPTIONAL == "true" ]]; then
   pip install -U -r requirements-optional.txt
 fi
 
+if [[ $SIX_VERSION != "false" ]]; then
+  pip install six==$SIX_VERSION
+fi
+
 if [[ $CI == "true" ]]; then
   pip install -U codecov
 fi

requirements.txt

@@ -1,3 +1,3 @@
-six
+six>=1.9
 webencodings
 ordereddict ; python_version < '2.7'

setup.py

@@ -102,7 +102,7 @@ def default_environment():
       maintainer_email='[email protected]',
       packages=find_packages(exclude=["*.tests", "*.tests.*", "tests.*", "tests"]),
       install_requires=[
-          'six',
+          'six>=1.9',
           'webencodings',
       ],
       extras_require={