Throw an exception when incompatible curos class is passed to SA engine (#272)

* Throw an exception when incompatible curos class is passed to SQLAlchemy engine

* flake8 fix

Author: David Jetelina

aiomysql/sa/engine.py

@@ -4,8 +4,9 @@
 
 import aiomysql
 from .connection import SAConnection
-from .exc import InvalidRequestError
+from .exc import InvalidRequestError, ArgumentError
 from ..utils import PY_35, _PoolContextManager, _PoolAcquireContextManager
+from ..cursors import Cursor
 
 
 try:
@@ -28,6 +29,11 @@ def create_engine(minsize=1, maxsize=10, loop=None,
     """
     coro = _create_engine(minsize=minsize, maxsize=maxsize, loop=loop,
                           dialect=dialect, pool_recycle=pool_recycle, **kwargs)
+    compatible_cursor_classes = [Cursor]
+    # Without provided kwarg, default is default cursor from Connection class
+    if kwargs.get('cursorclass', Cursor) not in compatible_cursor_classes:
+        raise ArgumentError('SQLAlchemy engine does not support '
+                            'this cursor class')
     return _EngineContextManager(coro)