Fix #1496 Async client uses blocking call when uploading file with v2

[seratch]

Summary

This pull request resolves #1496

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

Attention: Patch coverage is 38.46154% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 85.12%. Comparing base (694ec2f) to head (dd804f4).

Files	Patch %	Lines
slack_sdk/web/async_client.py	0.00%	4 Missing ⚠️
slack_sdk/web/client.py	0.00%	4 Missing ⚠️
slack_sdk/web/legacy_client.py	0.00%	4 Missing ⚠️
slack_sdk/web/async_base_client.py	50.00%	2 Missing ⚠️
slack_sdk/web/async_internal_utils.py	71.42%	2 Missing ⚠️
slack_sdk/web/base_client.py	50.00%	2 Missing ⚠️
slack_sdk/web/file_upload_v2_result.py	66.66%	2 Missing ⚠️
slack_sdk/web/internal_utils.py	0.00%	2 Missing ⚠️
slack_sdk/web/legacy_base_client.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1498      +/-   ##
==========================================
- Coverage   85.20%   85.12%   -0.08%     
==========================================
  Files         111      112       +1     
  Lines       12291    12316      +25     
==========================================
+ Hits        10472    10484      +12     
- Misses       1819     1832      +13     

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