chore: clean up project based on 3.6 removal

[WilliamBergamin]

Summary

These are minor changes that clean up the project based on python 3.6 support drop in #1735

Testing

The CI should cover the testing

Category

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs (Documents)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[zimeg]

@WilliamBergamin So fast! LGTM all throughout 🚢 💨

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.94%. Comparing base (64e1f30) to head (3c8122b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #1736 +/- ##
==========================================
+ Coverage 84.91% 84.94% +0.02% 
==========================================
Files 113 113 Lines 12895 12886 -9 ==========================================
- Hits 10950 10946 -4 + Misses 1945 1940 -5 

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