Fix negative TzOffset to_string handling

[sydney-runkle]

Fix pydantic/pydantic#8964

Our logic was a bit off in terms of converting negative timezone offsets -> the corresponding minute / second string representations.

[sydney-runkle]

Please review

[codecov]

Codecov Report

Merging #56 (a250007) into main (ef53708) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
- Coverage   99.12%   99.12%   -0.01%     
==========================================
  Files           6        6              
  Lines         918      917       -1     
==========================================
- Hits          910      909       -1     
  Misses          8        8              

Files	Coverage Δ
src/time.rs	97.80% <100.00%> (-0.01%)	⬇️

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