Fix milo output writing

[Zethson]

Fixes #819

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.58%. Comparing base (e56dbef) to head (77d23d2).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
pertpy/tools/_milo.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #821      +/-   ##
==========================================
- Coverage   72.58%   72.58%   -0.01%     
==========================================
  Files          47       47              
  Lines        5538     5540       +2     
==========================================
+ Hits         4020     4021       +1     
- Misses       1518     1519       +1     

Files with missing lines	Coverage Δ
pertpy/tools/_milo.py	79.76% <66.66%> (-0.18%)	⬇️

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