Fix #846 RTMClient does not pass timeout value to WebClient

[seratch]

Summary

This pull request fixes #846

Category (place an x in each of the [ ])

• slack.web.WebClient (Web API client)
• slack.webhook.WebhookClient (Incoming Webhook, response_url sender)
• slack.web.classes (UI component builders)
• slack.rtm.RTMClient (RTM client)
• Documents
• Others

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 #857 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #857   +/-   ##
=======================================
  Coverage   87.01%   87.01%           
=======================================
  Files          30       30           
  Lines        3797     3797           
  Branches      340      340           
=======================================
  Hits         3304     3304           
  Misses        344      344           
  Partials      149      149           

Impacted Files	Coverage Δ
slack/rtm/client.py	83.82% <ø> (ø)

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