Update version of twine and use twine check.

[tsroten]

Description

• Updates twine
• Uses twine check since python setup.py check -r is deprecated.
• Builds the source distribution in the packaging tests (so twine can check the description in it).
• Adds the missing content type metadata.

Checklist

• [x] I've added this contribution to the CHANGELOG.
• [x] I've added my name to the AUTHORS file (or it's already there).

[codecov-io]

Codecov Report

Merging #46 into master will increase coverage by 1.16%. The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #46      +/-   ##
========================================
+ Coverage   98.83%   100%   +1.16%     
========================================
  Files          11     11              
  Lines         429    429              
========================================
+ Hits          424    429       +5     
+ Misses          5      0       -5

Impacted Files	Coverage Δ
cli_helpers/config.py	100% <0%> (+4.5%)	:arrow_up:

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