[WIP] New dependency-alerter bot & support for async bot-handlers

[neiljp]

Motivation:

• Bot: It's challenging to keep up with latest updates of eg. mypy, so a bot seemed a reasonable idea :)
• Async: Grabbing the information via lots of http-requests takes time, as have other bots on occasion, so allowing for an async handler would enable the bots to be more responsive, to one user or between users.

Dependency-alerter bot:

• Looks at a requested URL for a requirements.txt-formatted file, then uses PyPI to check whether those dependencies are up to date.
• A dict of sample URLs are listed (zulip server repo!); open to including others as default :)
• Marked [WIP], since tests and docs are relatively limited so far.

Async support in lib.py:

• Appears to work well :)
• If the bot has an async handler, then it will try and run that instead of the regular one
• The lib runs the async event-loop in a new thread, and runs the bot async message-handler in that loop when a new message is received
• It shuts down the loop and thread upon SIGINT cleanly
• This will require python 3.5+, which I'm not sure if we specify explicitly (3.4 is maybe implied?)
• We could work with pre-3.5, but the syntax is somewhat less clean
• Python 3.6 would make things more concise again, but that may be going too far for now, at least for the lib
• [WIP] since there may be lib tests needing to be added?

Async support in dependency-alert bot:

• Initially retained use of sync requests, but with async def and await as appropriate
• Expansion of dict comprehension to for loop is required currently, assuming we stick with only python 3.5
• Last commit migrates to the async aiohttp from requests; this is faster due to the async nature

All commits should pass lint and run-mypy.

This is also WIP since I've not gone through the idea of async support on czo, but hopefully it seems reasonable, at least as a first step :)

Potential future bot developments:

• cache the requirements / pypi downloads on a 1h-1d basis
• allow the repo dict to be amended at run-time (or use storage)
• improve output of the current/available; maybe a table (though this can look a little ugly?)
• if a download isn't cached, post the message as eg. "processing..." and then update it later, given it can take a while (then the user gets feedback, and it places it after the user message, in time)

@showell - since async is bot library related @timabbott - discussed the bot with you on czo

[neiljp]

Perhaps unsurprisingly, this doesn't run pass the tests on python 3.4, though this seems as much about the presence of Awaitable in the annotations as the use of async.

[zulipbot]

Heads up @neiljp, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.