Fix #672 files.upload fails with a filepath containing multi byte chars

[seratch]

Summary

This pull request fixes #672 by changing the way to handle a local file path when uploading a file to the Slack server.

In addition, I've applied a minor improvement to use the local file name when the filename parameter is not given (currently this library set "file" for such cases).

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 #673 into master will decrease coverage by 0.08%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
- Coverage   87.17%   87.08%   -0.09%     
==========================================
  Files          15       15              
  Lines        1824     1827       +3     
  Branches      100      101       +1     
==========================================
+ Hits         1590     1591       +1     
- Misses        202      204       +2     
  Partials       32       32              

Impacted Files	Coverage Δ
slack/web/client.py	96.46% <33.33%> (-0.38%)	⬇️
slack/web/base_client.py	79.81% <100.00%> (ø)

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