Make --skip-existing work for Artifactory indexes

[mulmschneider]

Artifactory repositories seem to return HTTP 403 when you try to upload
a package that already exists (and your user doesn't have overwrite
permissions).
Extend skip_upload to support this.

[codecov]

Codecov Report

Merging #363 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #363      +/-   ##
==========================================
+ Coverage   68.84%   68.89%   +0.05%     
==========================================
  Files          12       12              
  Lines         597      598       +1     
  Branches       95       95              
==========================================
+ Hits          411      412       +1     
  Misses        158      158              
  Partials       28       28

Impacted Files	Coverage Δ
twine/commands/upload.py	66.27% <100%> (+0.39%)	⬆️

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[mulmschneider]

I only noticed just now that there is already an existing PR for exactly this functionality: #326. I'm perfectly fine with closing this one in favor of #326.

Let me know if there is anything I can do to help to get either of them merged.