Remove py36 references

[ewanek1]

Summary

This PR begins to resolve Issue 603. It removes explicit references and workarounds for Python 3.6 in the codebase and documentation to support minimum Python version 3.7+.

Testing

Category

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• 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 ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.96%. Comparing base (c75897e) to head (efbdb01).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1330   +/-   ##
=======================================
  Coverage   90.96%   90.96%           
=======================================
  Files         222      222           
  Lines        7501     7501           
=======================================
  Hits         6823     6823           
  Misses        678      678           

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