Update the way Attribute.slug field is generated

[Dresdn]

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Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc.)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Pull Request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Related issue(s)

Closes #223

Other Information

[codecov]

Codecov Report

Merging #227 (bbf12d5) into master (850ee17) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
+ Coverage   89.79%   89.98%   +0.18%     
==========================================
  Files          20       22       +2     
  Lines         745      759      +14     
  Branches      134      135       +1     
==========================================
+ Hits          669      683      +14     
  Misses         52       52              
  Partials       24       24              

Impacted Files	Coverage Δ
eav/fields.py	71.42% <ø> (-5.63%)	⬇️
eav/migrations/0001_initial.py	100.00% <ø> (ø)
eav/logic/slug.py	100.00% <100.00%> (ø)
eav/migrations/0008_use_native_slugfield.py	100.00% <100.00%> (ø)
eav/models.py	97.28% <100.00%> (+0.07%)	⬆️

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