Fix #690 by adding basic auth handling in the new sync client

[seratch]

Summary

This pull request fixes #690 by resolving the lack of basic auth header generation when run_async=False. This lack of implementation used to result in oauth.v2.access and oauth.access API call failures.

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 #691 into master will increase coverage by 0.19%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #691      +/-   ##
==========================================
+ Coverage   85.40%   85.59%   +0.19%     
==========================================
  Files          19       19              
  Lines        2624     2631       +7     
  Branches      249      252       +3     
==========================================
+ Hits         2241     2252      +11     
+ Misses        289      285       -4     
  Partials       94       94              

Impacted Files	Coverage Δ
slack/web/base_client.py	75.88% <42.85%> (+0.61%)	⬆️
slack/web/client.py	95.29% <0.00%> (+0.72%)	⬆️

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