Fix #714 by adding proxy support for sync WebClient

[seratch]

Summary

This pull request fixes a regression in v2.6.0 #714

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.

[codecov]

Codecov Report

Merging #715 into master will decrease coverage by 0.17%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master     #715      +/-   ##
==========================================
- Coverage   85.08%   84.91%   -0.18%     
==========================================
  Files          22       22              
  Lines        2783     2790       +7     
  Branches      277      279       +2     
==========================================
+ Hits         2368     2369       +1     
- Misses        312      317       +5     
- Partials      103      104       +1     

Impacted Files	Coverage Δ
slack/web/base_client.py	70.73% <14.28%> (-1.30%)	⬇️

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