Fix aiomysql.sa in Python 3.7 and run tests for python 3.7 in CI

.travis.yml

@@ -1,8 +1,10 @@
+dist: xenial
 language: python
 
 python:
     - 3.5.3
     - 3.6
+    - 3.7
 
 env:
     matrix:
@@ -11,22 +13,27 @@ env:
 
 services:
     - docker
+    - mysql
 
 matrix:
    include:
         - python: 3.6
+          dist: trusty
           env: PYTHONASYNCIODEBUG=
           addons:
               mariadb: 5.5
         - python: 3.6
+          dist: trusty
           env: PYTHONASYNCIODEBUG=1
           addons:
               mariadb: 10.0
         - python: 3.6
+          dist: trusty
           env: PYTHONASYNCIODEBUG=
           addons:
               mariadb: 10.1
         - python: 3.6
+          dist: trusty
           env: PYTHONASYNCIODEBUG=
           addons:
               mysql: 5.7

aiomysql/utils.py

@@ -70,9 +70,19 @@ async def __aexit__(self, exc_type, exc, tb):
 
 
 class _SAConnectionContextManager(_ContextManager):
-    async def __aiter__(self):
-        result = await self._coro
-        return result
+    def __aiter__(self):
+        return self
+
+    async def __anext__(self):
+        if self._obj is None:
+            self._obj = await self._coro
+
+        try:
+            return (await self._obj.__anext__())
+        except StopAsyncIteration:
+            await self._obj.close()
+            self._obj = None
+            raise
 
 
 class _TransactionContextManager(_ContextManager):