Fixes --skip-existing with Nexus Repos

[superwhoopy]

Small fix for the option --skip-existing when uploading to a Nexus Repository.

• Ran tox -e lint
• Ran tests

Thank you for your great work on twine!

[codecov]

Codecov Report

Merging #428 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #428   +/-   ##
=======================================
  Coverage   78.29%   78.29%           
=======================================
  Files          14       14           
  Lines         751      751           
  Branches      108      108           
=======================================
  Hits          588      588           
  Misses        130      130           
  Partials       33       33

Impacted Files	Coverage Δ
twine/commands/upload.py	75% <100%> (ø)	⬆️

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[theacodes]

Thank you, @superwhoopy!