Replace dateutil.relativedelta using stdlib to drop six

[hugovk]

We were only using dateutil.relativedelta in one place, so we can drop the dependency and say goodbye to its dependency six.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.06%. Comparing base (698ae20) to head (a4d7bb4).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #267      +/-   ##
==========================================
- Coverage   99.06%   99.06%   -0.01%     
==========================================
  Files           7        7              
  Lines         430      429       -1     
==========================================
- Hits          426      425       -1     
  Misses          4        4              

Flag	Coverage Δ
macos-latest	99.06% <100.00%> (-0.01%)	⬇️
ubuntu-latest	99.06% <100.00%> (-0.01%)	⬇️
windows-latest	99.06% <100.00%> (-0.01%)	⬇️

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