py/compile: Fix async-for/async-with to work with simpler exc on stack.

There is now just the exception instance on the stack when an exception is
raised, not the full (type, exc, traceback).

tests/basics/async_with.py

@@ -4,7 +4,7 @@ class AContext:
     async def __aenter__(self):
         print('enter')
     async def __aexit__(self, exc_type, exc, tb):
-        print('exit')
+        print('exit', exc_type, exc)
 
 async def f():
     async with AContext():
@@ -15,3 +15,13 @@ try:
     o.send(None)
 except StopIteration:
     print('finished')
+
+async def g():
+    async with AContext():
+        raise ValueError('error')
+
+o = g()
+try:
+    o.send(None)
+except ValueError:
+    print('ValueError')