Update slackclient requirement from <2,>=1.2.1 to >=1.2.1,<3

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Updates the requirements on slackclient to permit the latest version.

Release notes

Sourced from slackclient's releases.

Slack Client v2 Official Release

Original RFC: #384
v2 PR: #394

New Features

1. Client Decomposition: We’ve split the client into two.
   1. WebClient: A HTTP client focused on Slack's Web API.
   2. RTMClient: A websocket client focused on Slack's RTM API.
2. RTMClient: Completely redesigned, this client allows you to link your application's callbacks to corresponding Slack events.
3. WebClient: The WebClient now provides built-in methods for Slack's Web API. These methods act as helpers enabling you to focus less on how the request is constructed. Here are a few things that this provides:
   1. Basic information about each method through the docstring.
   2. Easy File Uploads: You can now pass in the location of a file and the library will handle opening and retrieving the file object to be transmitted.
   3. Token type validation: This gives you better error messaging when you're attempting to consume an api method that your token doesn't have access to.
   4. Constructs requests using Slack's preferred HTTP methods and content-types.

Breaking Changes:
If you're migrating from v1.x of slackclient to v2.x, Please follow our migration guide to ensure your app continues working after updating.

Check out the Migration Guide here!

Commits

• ee78eed Bumping version and updating Readme (#404)
• 424f840 Correcting sizing of text in tutorial
• 985f873 Updating tutorial with the instructions to start.
• 901341c Updating the install version on ReadMe.
• a306173 Correcting version for tutorial
• 852517c Bumping version
• 72f4f1e Slack client v2 development (#394)
• 66f1fb5 Return an empty dict when response is empty instead of throwing an error (#395)
• d6190cc 1.3.1 (#388)
• d89893a Lock websocket-client version: temp fix for #385 (#387)
• Additional commits viewable in compare view

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