Switch slack library from slacker-asyncio to aioslacker

[jacobtomlinson]

Given all the problems we've had with slacker-asyncio I thought I'd try out aioslacker instead.

The main benefits are:

• Maintained by the aio-libs org
• Doesn't override the synchronous slacker module
• Imports slacker and overrides methods which should be more future compatible
• Has more flexible dependency specifications

Main drawbacks are:

• Looks like is hasn't been maintained as recently
• Currently uses an older version of slacker

Would be interested in thoughts from @FabioRosado and @Cadair

[FabioRosado]

To be fair our slack connector just needs to get and deliver messages from a slack channel (since we aren't supporting other types of message yet) so I think the fact that aioslacker lib is using an older version os slacker might not pose too much of an issue.

I also haven't received any answer on the issue that I created from the creator of slacker-asyncio so I'm not sure if this lib is actually being maintained actively.

For what you said about the benefits of aioslacker seems like a good way for us to mitigate the issue with the dependency hell issue and if @Cadair had problems with slacker-asyncio overwriting slacker I'm sure someone else will have the same issue in the future.

Basically, this is a big text to say that I might be a good idea to change from slacker-asyncio to aioslacker 👍

[codecov]

Codecov Report

Merging #597 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #597   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          20     20           
  Lines        1369   1369           
=====================================
  Hits         1369   1369

Impacted Files	Coverage Δ
opsdroid/connector/slack/__init__.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8160a8a...850f374. Read the comment docs.

[FabioRosado]

Should we merge this and see how everything works or would you like me to give some quick testing on my end?
Also sorry been a bit busy with visiting family and friends - plus trying to survive the 45 degrees @.@

[jacobtomlinson]

I want to do a bit of manual testing. Feel free to do some too.

Don't worry, I just try and do things when I can.

[Cadair]

👍 This looks like it would solve my issue with the namespace overwriting. Other than that I don't use opsdroid with slack apart from one project (and then I am using slacker directly) so I can't be much help testing. :)