Slack-sdk

**New Features**
1. [WebClient] Adding new admin and remote files API methods. 501
2. [WebClient] Adding new view API methods. 517

**Updates**
1. [Message Builders] Update BlockAttachment to not send invalid JSON due to fields attribute 473 (Thanks paul-griffith)
2. [Docs] Add RTM section for docs v2 477 (Thanks shanedewael)
3. [Docs] Fix typo; recieved -> received 478 (Thanks joakimnordling)
4. [Docs] Fix block kit link & update docs 484 (Thanks clavin)
5. [RTMClient] Return callback from `RTMClient.run_on` 490 (Thanks clavin)
6. [Docs] Fix link to Auth Guide in readme 498 (Thanks asherf)
7. [Docs] Fix missing word and typo 512 (Thanks marks)
8. [Message Builders] bugfix for value length in button elements 514 (Thanks avanderm)
9. [Docs] Fixes formatting 515 (Thanks vpetersson)
10. [Docs] Improve a code snippet on README 516 (Thanks seratch)
11. [WebClient] Fixed an OAuth Headers bug and made the `token` param optional. 517

**New Features**
1. Type-hinted helper classes for building messages in v2 400 (Thanks paul-griffith)

**Breaking Changes**
1. [RTMClient] Converted the `RTMClienttyping()` function to async 446

**Updates**
1. [RTMClient] Handle case in which aiohttp closes the websocket due to lack of ping responses. 453 (Thanks flyte)
2. Modify package identifier in user agent to match v1.x identifier 418 (Thanks aoberoi)
3. [WebClient] Fixed typo in Scheduled message 428 & 435 (Thanks splinterific)
4. Transform install_requires of 'aiodns' into extras_require. 440 (Thanks staticdev)

**Thank you!!**
To everyone who's opened, commented or reacted to an issue; this project is better because of you!
Thank you for helping the Slack community!

Original RFC: https://github.com/slackapi/python-slackclient/issues/384
v2 PR: https://github.com/slackapi/python-slackclient/pull/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!](https://github.com/slackapi/python-slackclient/wiki/Migrating-to-2.x)

Fixing an issue where reconnects used rtm.start istead of rtm.connect. 422

- Locking websocket-client version to < 0.55.0: temp fix for 385

New Features
- Adds support for short lived tokens and automatic token refresh 347 (Thanks roach!)

Other
- update RTM rate limiting comment and error message 308 (Thanks benoitlavigne!)
- Use logging instead of traceback 309 (Thanks harlowja!)
- Remove Python 3.3 from test environments 346 (Thanks roach!)
- Enforced linting when using VSCode. 347 (Thanks roach!)