Migrate deprecated codecov to the latest recommended way

[seratch]

Summary

About 12 hours ago, codecov package was deleted from the PyPI registry as it's no longer supported. This pull request migrates the way to upload the coverage data to the service. In addition to the CI job setting change, I've added a new secret named CODECOV_TOKEN to this repository.

References:

• https://github.com/codecov/codecov-python
• https://community.codecov.com/t/codecov-yanked-from-pypi-all-versions/4259

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.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-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• 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.

[codecov]

Codecov Report

Merging #1353 (1010771) into main (ee23290) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 1010771 differs from pull request most recent head 540e315. Consider uploading reports for the commit 540e315 to get more accurate results

@@            Coverage Diff             @@
##             main    #1353      +/-   ##
==========================================
- Coverage   85.38%   85.37%   -0.01%     
==========================================
  Files         111      111              
  Lines       11556    11556              
==========================================
- Hits         9867     9866       -1     
- Misses       1689     1690       +1     

see 1 file with indirect coverage changes

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