Add certifi fallback for SSL certificate issues

[monolith-jaehoon]

Using Claude 4 Sonnet

Summary

• Add certifi fallback for SSL certificate issues
• Relate #1491
• Using certifi if only existed certifi

Testing

• Install Windows 11 and remain offline from internet
  • Or remove ISRG Root X1 certificate in OS
• Install and Use Slack SDK

Category

• [x] slack_sdk.web.WebClient (sync/async) (Web API client)
• [x] 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

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

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @monolith-jaehoon to sign the Salesforce Inc. Contributor License Agreement.