refactor: Simplify yearbook data generation

[robinholzi]

Motivation

As yearbook is a rather small dataset, I propose to always store the train, test and all (new) variants.

Changes

• Always output all three splits of the datasets
• Use pathlib instead of os.path for cleaner code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.34%. Comparing base (beb3769) to head (fee28d5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #523   +/-   ##
=======================================
  Coverage   82.34%   82.34%           
=======================================
  Files         216      216           
  Lines       10085    10085           
=======================================
  Hits         8304     8304           
  Misses       1781     1781           

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[github-actions]

^{( % to main)
( % to main)}

[MaxiBoether]

👍 (if CI runs through)