Rewrite to use asyncio instead of asynchat

[dgw]

Python's asyncore and asynchat modules are deprecated as of 3.6. The asyncio module was added in 3.4, and Sopel should probably be rewritten on top of that when Python 2.x support is dropped.

I don't think there's a specific timeline for that happening yet. Maybe whenever Python releases a stable version that no longer supports asyncore and asynchat, it'll be a good time to make Sopel require Python 3.4+. That probably won't happen for years to come, but I think it's good to get this on the radar.

No milestone until Python actually removes the modules. The next major release of Sopel after that happens will include this rewrite. (Anyone want to place bets on what the version number will be? 😹)

[dgw]

(I totally didn't decide to open this issue because I noticed #1414 was the next number. Nope… /s)

[Exirel]

Some information about versions:

• asyncio was included in Py3.4 as a previsional API, and is stable since Py3.6 only (so no Py3.5 here),
• asynchat is deprecated since Py3.6
• Py3.4 last release might be around March 2019, that will be the last release for Py3.4,
• Py2.7 end of life is planned for January 2020

Looking at these dates, what I suggest is to:

• drop py2.7 and py3.4 support January 2020,
• keep asynchat until the drop of Py3.5 support,

and as a side note: I worked on a prototype of IRC bot using asyncio, and it isn't that complicated, but it requires careful planning and testing.

[Exirel]

With #1697 it will be possible to implement a different backend for asyncio - or any kind of backend. So all that's left to do now is to actually implement one!

[Exirel]

So, after a few hours of tinkering with asyncio and working on a new backend, I've come to some conclusions:

1. It's doable: I've got a running bot that can initialize the connection, start communication with the server, join channels, and react to commands (both with and without threads, which is great).
2. I still need to iron out the proper workflow of connection, disconnection, when to stop reading, how and when to properly quit, i.e. I need to understand and properly implement the loop lifecycle
3. I think we'll be able, moving forward, to take advantage of Python 3.7+ asyncio interface to slowly transform Sopel's plugin handlers into coroutines, while being fully backward compatible (thanks to some magic with Task, Future, and so many little things)
4. I still need some time, but we are getting there, and it's nice!

It's hard, but damn that's interesting!

[Exirel]

1. it's not only running, it's also running with SSL on!
2. ping timeout, server timeout, and signal handlers are all done!
3. I think I know how to, but that'll be for another issue, one day (maybe even Sopel 8.1 or 9?)
4. There is now a draft PR, see bot, irc: IRC backend using asyncio #2256