Add support for Slack shared secret message signing

[Latent-Logic]

Support was added to the underlying Slack library in pyslackers/slack-sansio#41
It is the recommended way to verify requests from Slack now:
https://api.slack.com/docs/verifying-requests-from-slack

Also when running the tests I ran into aio-libs/aiohttp#2578 so I fixed those deprecation warnings.

[Latent-Logic]

The Travis black failure is coming from line 117, which isn't even changed in my commit:

-                "updated": 1502138686,
+                "updated": 1_502_138_686,

(This issue didn't show up when running tox locally)

[ovv]

I don't recall black formatting number that way. It's probably due to a newer version.

[Latent-Logic]

Travis succeeded for everything except Python 2.6, however that matches the pattern from the Travis run a few days ago on commit 10656ed:
https://travis-ci.org/pyslackers/sir-bot-a-lot-2/builds/459096989

The error specifically has to do with a failure to import idna_ssl which is included by aiohttp

[ovv]

Looks very good. Thanks for this. I'll take a deeper look & do some tests as soon as possible.

The failure is due to idna_ssl being required for python 3.6 but not required for python 3.7 and pipenv doesn't handle that well for the moment. I migrated slack-sansio to poetry a few days ago, I'll probably do the same for this repo.

[ovv]

Two small things and then we can

[ovv]

That code is used in multiple endpoint. I would be best to take it out in a function, something like

try:
    validate_request(request, slack.signing_secret)
    event = Event.from_http(payload, verification_token=slack.verify)
except (FailedVerification, InvalidSlackSignature, InvalidTimestamp):

[Latent-Logic]

I chose to explicitly keep verification_token as a new variable so the token we're passing into Event is explicitly None when the signing_secret is set. This way we're not relying on the initialization behavior in slack/plugin.py. If we wanted to rely on no one tampering with the slack.verify parameter then we could just call validate_request_signature rather than wrapping it in a validate_request function.