Fix #1000 RTMClient v2 still requires aiohttp installed (even though it's unused)

[seratch]

Summary

This pull request resolves #1000 by adding slack_sdk.rtm_v2.RTMClient as the default way to use the v2 RTM client. We still continue supporting slack_sdk.rtm.v2 as an alias for this module.

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.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)
• /docs-src (Documents, have you run ./docs.sh?)
• /docs-src-v2 (Documents, have you run ./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 python setup.py validate after making the changes.

[codecov]

Codecov Report

Merging #1001 (4f3faff) into main (aebf6db) will increase coverage by 0.03%.
The diff coverage is 52.08%.

@@            Coverage Diff             @@
##             main    #1001      +/-   ##
==========================================
+ Coverage   86.58%   86.62%   +0.03%     
==========================================
  Files          94       95       +1     
  Lines        8747     8748       +1     
==========================================
+ Hits         7574     7578       +4     
+ Misses       1173     1170       -3     

Impacted Files	Coverage Δ
slack_sdk/rtm_v2/__init__.py	51.88% <51.88%> (ø)
slack_sdk/rtm/v2/__init__.py	100.00% <100.00%> (+48.11%)	⬆️
slack_sdk/socket_mode/client.py	76.99% <0.00%> (+2.65%)	⬆️

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[mwbrooks]

Looks good and nice to see a few rocket ship reactions 🚀

This is part of v3.5.0 because it's a breaking change from due to changing the import path?

[seratch]

@mwbrooks

This is part of v3.5.0 because it's a breaking change from due to changing the import path?

Let me clarify this a bit more.

This is not a breaking change to any existing apps depending on slack_sdk.rtm.v2 package. As long as it works for the apps, they don't need to change imports at all. The package still exists and works as like before.

The new source package slack_sdk.rtm_v2 enables new developers (or the ones who are currently wanting to remove aiohttp from their apps) not to have aiohttp only for the reason where slack_sdk.rtm.v2 package requires it.