python37 type check fix

.travis.yml

@@ -6,6 +6,7 @@ python:
     - "pypy3.5"
     - "3.5"
     - "3.6"
+    - "3.7-dev"
 
 install:
     - pip install -r requirements.txt

apistar/server/core.py

@@ -88,9 +88,10 @@ def generate_response(self, handler):
         return Response(encoding='application/json', status_code=200, schema=annotation)
 
     def coerce_generics(self, annotation):
+        origin = getattr(annotation, '__origin__', annotation)
         if (
-            isinstance(annotation, type) and
-            issubclass(annotation, typing.List) and
+            isinstance(origin, type) and
+            issubclass(origin, typing.List) and
             getattr(annotation, '__args__', None) and
             issubclass(annotation.__args__[0], types.Type)
         ):