add python3.6 tests typechecking

.travis.yml

@@ -10,10 +10,14 @@ jobs:
         set -e
         pytest
 
-    - name: Typecheck Django test suite
+    - name: Typecheck Django test suite with python 3.7
       python: 3.7
       script: 'python ./scripts/typecheck_tests.py'
 
+    - name: Typecheck Django test suite with python 3.6
+      python: 3.6
+      script: 'python ./scripts/typecheck_tests.py'
+
     - name: Lint with black
       python: 3.7
       script: 'black --check --line-length=120 django-stubs/'