fix work with 0.21 without patching aiohttp

Dependent aiohttp changes are here:
<aio-libs/aiohttp#767>.

aiohttp_cors/cors_config.py

@@ -164,11 +164,16 @@ def add(self,
         :return: ``route``.
         """
 
-        if _AIOHTTP_0_21 and isinstance(route, web.AbstractResource):
-            # TODO: Resources should be supported.
-            raise RuntimeError(
-                "You need to pass Route to the CORS config, and you passed "
-                "Resource.")
+        if _AIOHTTP_0_21:
+            # TODO: Use web.AbstractResource when this issue will be fixed:
+            # <https://github.com/KeepSafe/aiohttp/pull/767>
+            from aiohttp.web_urldispatcher import AbstractResource
+
+            if isinstance(route, AbstractResource):
+                # TODO: Resources should be supported.
+                raise RuntimeError(
+                    "You need to pass Route to the CORS config, and you "
+                    "passed Resource.")
 
         if route in self._preflight_route_settings:
             _logger.warning(

aiohttp_cors/urldispatcher_router_adapter.py

@@ -70,14 +70,18 @@ class _UrlDispatcherRouterAdapter_v21(AbstractRouterAdapter):
     def __init__(self, router: web.UrlDispatcher):
         self._router = router
 
-    def route_methods(self, route: web.AbstractRoute):
+    def route_methods(self, route):
         """Returns list of HTTP methods that route handles"""
         return [route.method]
 
-    def add_options_method_handler(self, route: web.AbstractRoute, handler):
+    def add_options_method_handler(self, route, handler):
         method = "OPTIONS"
 
-        if isinstance(route, web.ResourceRoute):
+        # TODO: Use web.ResourceRoute when this issue will be fixed:
+        # <https://github.com/KeepSafe/aiohttp/pull/767>
+        from aiohttp.web_urldispatcher import ResourceRoute
+
+        if isinstance(route, ResourceRoute):
             # Route added through Resource API.
             new_route = route.resource.add_route(method, handler)