refactor: Use datetime.fromisoformat in other places

[edgarrmondragon]

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📚 Documentation preview 📚: https://meltano-sdk--2092.org.readthedocs.build/en/2092/

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (fe0ff85) 87.70% compared to head (94b99e3) 87.71%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
singer_sdk/testing/tap_tests.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2092      +/-   ##
==========================================
+ Coverage   87.70%   87.71%   +0.01%     
==========================================
  Files          60       60              
  Lines        4937     4942       +5     
  Branches     1005     1007       +2     
==========================================
+ Hits         4330     4335       +5     
  Misses        425      425              
  Partials      182      182              

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[codspeed-hq]

CodSpeed Performance Report

Merging #2092 will not alter performance

_{Comparing edgarrmondragon/refactor/more-datetime.fromisoformat (94b99e3) with main (fcf30d5)}

Summary

✅ 4 untouched benchmarks