py/compile: Fix handling of unwinding BaseException in async with.

All exceptions that unwind through the async-with must be caught and
BaseException is the top-level class, which includes Exception and others.

Fixes issue #4552.

tests/basics/async_with.py

@@ -27,3 +27,13 @@ try:
     o.send(None)
 except ValueError:
     print('ValueError')
+
+# test raising BaseException to make sure it is handled by the async-with
+async def h():
+    async with AContext():
+        raise BaseException
+o = h()
+try:
+    o.send(None)
+except BaseException:
+    print('BaseException')