Show proper yaml syntax in resolve_config deprecation warnings containing None

[vmichals]

Description

Fix #390

Changes

Instead of printing the repr(), the suggested change is now passed through yaml.dump()

Checklist

• does the chosen indentation fit all/most use cases?

Quality

• I have read the CONTRIBUTING doc
• My commits messages follow this format
• My code follows the style guidelines ($ tox -e lint)

[codecov-commenter]

Codecov Report

Merging #391 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #391   +/-   ##
========================================
  Coverage    47.30%   47.30%           
========================================
  Files           67       67           
  Lines        12932    12932           
  Branches       321      321           
========================================
  Hits          6117     6117           
  Misses        6790     6790           
  Partials        25       25           

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[Thomsch]

Thank you for your contribution @vmichals !

[bouthilx]

Closing and re-opening to try fixing travis ci issue. I don't know why it's appearing twice here.