Add types for six

.github/workflows/lint.yml

@@ -8,10 +8,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.7
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.7
+        python-version: 3.9
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip

tox.ini

@@ -32,6 +32,7 @@ commands =
 [testenv:mypy]
 deps =
     mypy
+    types-six
 commands =
     mypy graphene