Fix Build Error

[stasfilin]

Summary

Fix flake8 errors.

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.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3ccd408). Click here to learn what that means.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             master     #245   +/-   ##
=========================================
  Coverage          ?   59.62%           
=========================================
  Files             ?        9           
  Lines             ?      317           
  Branches          ?       58           
=========================================
  Hits              ?      189           
  Misses            ?      113           
  Partials          ?       15

Impacted Files	Coverage Δ
slackclient/client.py	37.93% <0%> (ø)
slackclient/server.py	58.77% <100%> (ø)

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