[FIX] OpenAPI3 Killmails not serializable

[Geuthur]

Description

Please include a summary of the changes and the related issue number (if applicable).

Fixed

• OpenAPI3 Killmails not serializable

Type of Changes

Please select the type of changes made in this pull request:

• Bug Fix (non-breaking change which fixes an Issue)
• New Feature (non-breaking change)
• Other (please specify):

Checklist

Please ensure the following before submitting your pull request:

• I have read the contributing guidelines.
• My code follows the code style of this project.
• I have performed a self-review of my code.
• I have commented my code where necessary.
• All new and existing tests passed.

[codecov]

Codecov Report

❌ Patch coverage is 15.38462% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
killstats/helpers/killmail.py	15.39%	11 Missing ⚠️

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   54.93%   54.62%   -0.31%     
==========================================
  Files          26       26              
  Lines        1249     1258       +9     
  Branches      115      119       +4     
==========================================
+ Hits          686      687       +1     
- Misses        555      563       +8     
  Partials        8        8              

Files with missing lines	Coverage Δ
killstats/helpers/killmail.py	38.74% <15.39%> (-0.70%)	⬇️

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