Update websockets dependency requirements

Hey!



I have been using this SDK with a uvicorn server. They have recently upgraded the websockets requirements ([link](https://github.com/encode/uvicorn/pull/1065)), and the version 9.1 includes a fix for a security issue introduced in version 8. 



Currently, they require `>=9.1`, while this SDK requires `>=8,<9`.  It would be awesome if you could also bump your requirements so that we didn't need to pin the uvicorn version and could also benefit from this security fix.



I'd be happy to open a PR to help with the upgrade if needed, but I couldn't quite figure out how the library is used and what code changes might need to be done for this major upgrade.



### Category (place an `x` in each of the `[ ]`)



- [ ] **slack_sdk.web.WebClient (sync/async)** (Web API client)

- [ ] **slack_sdk.webhook.WebhookClient (sync/async)** (Incoming Webhook, response_url sender)

- [ ] **slack_sdk.models** (UI component builders)

- [ ] **slack_sdk.oauth** (OAuth Flow Utilities)

- [X] **slack_sdk.socket_mode** (Socket Mode client)

- [ ] **slack_sdk.audit_logs** (Audit Logs API client)

- [ ] **slack_sdk.scim** (SCIM API client)

- [ ] **slack_sdk.rtm** (RTM client)

- [ ] **slack_sdk.signature** (Request Signature Verifier)



### Requirements



Please read the [Contributing guidelines](https://github.com/slackapi/python-slack-sdk/blob/main/.github/contributing.md) and [Code of Conduct](https://slackhq.github.io/code-of-conduct) before creating this issue or pull request. By submitting, you are agreeing to those rules.

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