generalize scoped_session proxying and apply to asyncio elements

Reworked the proxy creation used by scoped_session() to be
based on fully copied code with augmented docstrings and
moved it into langhelpers.  asyncio session, engine,
connection can now take
advantage of it so that all non-async methods are availble.

Overall implementation of most important accessors / methods
on AsyncConnection, etc. , including awaitable versions
of invalidate, execution_options, etc.

In order to support an event dispatcher on the async
classes while still allowing them to hold __slots__,
make some adjustments to the event system to allow
that to be present, at least rudimentally.

Fixes: #5628
Change-Id: I5eb6929fc1e4fdac99e4b767dcfd49672d56e2b2

1 files changed, 2 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/events.py b/lib/sqlalchemy/orm/events.py
index 29a509cb9..4e11ebb8c 100644
--- a/lib/sqlalchemy/orm/events.py
+++ b/lib/sqlalchemy/orm/events.py
@@ -1371,7 +1371,8 @@ class SessionEvents(event.Events):
         elif isinstance(target, Session):
             return target
         else:
-            return None
+            # allows alternate SessionEvents-like-classes to be consulted
+            return event.Events._accept_with(target)
 
     @classmethod
     def _listen(cls, event_key, raw=False, restore_load_context=False, **kw):