Use pydantic alias for planned maintenance fields

[johannaengland]

Scope and purpose

Fixes #287.

Contributor Checklist

Every pull request should have this checklist filled out, no matter how small it is.
More information about contributing to Zino can be found in the
README.

• Added a changelog fragment for towncrier
• Added/amended tests for new/changed code
• Added/changed documentation
• Linted/formatted the code with black, ruff and isort, easiest by using pre-commit
• The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See https://cbea.ms/git-commit/
• If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.32%. Comparing base (395ac8e) to head (40250d8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #328   +/-   ##
=======================================
  Coverage   97.32%   97.32%           
=======================================
  Files          64       64           
  Lines        7919     7920    +1     
=======================================
+ Hits         7707     7708    +1     
  Misses        212      212           

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