fix: move away from datetime.utcfromtimestamp for the state and installation stores

[WilliamBergamin]

Summary

This PR aims to resolve #1676 by favoring the following

• datetime.utcfromtimestamp(...) -> datetime.fromtimestamp(..., tz=timezone.utc)
• datetime.utcnow() -> datetime.now(tz=timezone.utc)

Some investigation was required to ensure this change would not impact existing application. Since the timestamps are stored and compared as integers this change should not cause any breaking change.

Testing

Unit test coverage should be sufficient

Category

• 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 (Documents)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements

• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.91%. Comparing base (4601f8a) to head (d7caaf4).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1798   +/-   ##
=======================================
  Coverage   83.90%   83.91%           
=======================================
  Files         115      115           
  Lines       13080    13080           
=======================================
+ Hits        10975    10976    +1     
+ Misses       2105     2104    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[zimeg]

Super nice changes! I left a comment around the effects of these changes and I'm also curious if #1731 might be addressed with this too?

I'm marking this as "approved" but do let me know if more review and testing can be helpful 🙏 ✨

[zimeg]

🤩 praise: @WilliamBergamin I'm a huge fan of keeping all representations of time in the UTC timezone!

👁️‍🗨️ question: Would adjacent implementations also require a similar change? I find this initial value might be generated with an offset:

python-slack-sdk/slack_sdk/oauth/installation_store/models/installation.py

Lines 131 to 134 in d7caaf4

	if installed_at is None:
	self.installed_at = datetime.now().timestamp()
	else:
	self.installed_at = _timestamp_to_type(installed_at, float)

📚 https://docs.python.org/3/library/datetime.html#datetime.datetime.now