Asyncio send mail backend

[Andrew Chen Wang]

Implementation of asyncio send mail functionality. All methods get its a prefix variant, and the get_connection runs a start command unlike its sync version.

​https://github.com/django/django/pull/16744

Adds Base, Console, and Dummy backends (no test; pending review of draft implementation).

[Mariusz Felisiak]

Accepted based on ​DEP-9.

[Ahmed Ibrahim]

I will handle this since it became inactive and I think it's a neat feature

[Ahmed Ibrahim]

Would it be better if I leave the sync implementations as is, and add a new async implementation, because I my mind tells that we can create the async and use the async to create a new sync handler with sync_to_async so that the logic is only in one place

[Mike Edmunds]

Replying to Ahmed Ibrahim:

Would it be better if I leave the sync implementations as is, and add a new async implementation, because I my mind tells that we can create the async and use the async to create a new sync handler with sync_to_async so that the logic is only in one place

My instinct is that each EmailBackend should decide for itself whether to implement send_messages(), asend_messages(), or both. And then maybe the BaseEmailBackend implementation could sync_to_async or async_to_sync if a subclass only provides one of them. (But I'd need to think some more about what happens if an existing sync send method gets asyncified.)