functions: Use Slack client lib for request validation

[ace-n]

N.B: this adds an additional package (slackclient) to do signature verification. (The code readability/maintainability benefits likely outweigh the drawbacks of adding an additional package.)

[grant]

I really think we can't support Slack samples in our repos and we should move them to community samples. Adding slack dependencies like this isn't going to be long term sustainable with our other samples and languages.

[engelke]

First, fix lint errors.

Why is slackeventsapi in requirements.txt? It doesn't seem to be imported into any code. The corresponding Node.js sample uses a similarly named library in the signature verification (https://cloud.google.com/functions/docs/tutorials/slack#functions_slack_setup_tutorial-nodejs) instead of slackclient.

The prior version code does not seem too complex, and would show readers trying to use a different language the actual operations needed. Consider whether this change is good for all readers or not.

I don't feel strongly about whether to use the new slack library or not. So long as this sample is used in a tutorial on the website, the code does belong here.

[ace-n]

@grant this tutorial was written quite awhile ago - likely before GSuite Chat was released. I'm potentially open to replacing it with one based around GSuite Chat.

@engelke I agree with your concern RE making this similar to other language samples. (Even if those languages have their own samples, this Python-specific implementation may come off as arbitrary.) Which would you (and @grant) say is more important here - consistency, or simpler code? (Personally I'm on the fence - I see value in both.)

[ace-n]

Ping on this. 🙂

[engelke]

LGTM