Use modern Python syntax features

[jdufresne]

• Can omit ordered .format() placeholder numbers
• Use set literals
• Use dict comprehension

Changes automated with the pyupgrade tool:

https://github.com/asottile/pyupgrade

[codecov]

Codecov Report

Merging #436 into master will not change coverage.
The diff coverage is 70%.

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  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/register.py	0% <0%> (ø)	⬆️
twine/cli.py	100% <100%> (ø)	⬆️
twine/commands/upload.py	75% <100%> (ø)	⬆️
twine/exceptions.py	100% <100%> (ø)	⬆️
twine/package.py	86.72% <100%> (ø)	⬆️
twine/utils.py	83.68% <100%> (ø)	⬆️
twine/repository.py	60.57% <33.33%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #436 into master will not change coverage.
The diff coverage is 70%.

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  Coverage   78.29%   78.29%           
=======================================
  Files          14       14           
  Lines         751      751           
  Branches      108      108           
=======================================
  Hits          588      588           
- Misses        129      130    +1     
+ Partials       34       33    -1

Impacted Files	Coverage Δ
twine/commands/register.py	0% <0%> (ø)	⬆️
twine/cli.py	100% <100%> (ø)	⬆️
twine/commands/upload.py	75% <100%> (ø)	⬆️
twine/exceptions.py	100% <100%> (ø)	⬆️
twine/package.py	86.72% <100%> (ø)	⬆️
twine/utils.py	83.68% <100%> (ø)	⬆️
twine/repository.py	60.57% <33.33%> (ø)	⬆️
twine/wininst.py	31.57% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6c4d5ec...869392b. Read the comment docs.