Drop Support for EOL Python 3.6, 3.7 and 3.8

As of today per Python Software Foundation policy here: https://devguide.python.org/versions/

• Python 3.6 has ended security support and it's end of life was - 2021-12-23. It's 3 years unsupported!
• Python 3.7 has ended security support and it's end of life was - 2023-06-27. It's 1.5 years unsupported!
• Python 3.8 has ended security support and it's end of life was - 2024-10-07. It's 3 months unsupported!

Does Slack SDK plans to remove support for those Python versions? Promoting unsupported version of Python has security concerns and it shouldn't be promoted.

I've realised this while working on #1633 as I wasn't expecting that Python 3.6 is in use and I was using features that were added in Python 3.8 so I lived under assumption that I can use everything from 3.9 and above.

Category

• 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)
• 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 and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

No se nombran archivos, pruebas ni puntos de entrada. Empieza revisando la configuración del proyecto y las referencias de CI a las versiones de Python compatibles; después, inspecciona los componentes del SDK indicados para comprobar la compatibilidad específica de cada versión. Se considera terminado cuando Python 3.6, 3.7 y 3.8 dejen de ser compatibles de forma coherente en esos componentes y se actualice la validación correspondiente.