Fix deprecation warning

[DanielNoord]

Was looking at an issue report and saw we had a DeprecationWarning in the tests.

Let's fix it, and also make sure we get warned about them :)

[github-actions]

According to the primer, this change has no effect on the checked open source code. 🤖🎉

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (13d4261) to head (a54eafb).
⚠️ Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #490   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           31        31           
  Lines         1126      1126           
  Branches       120       120           
=========================================
  Hits          1126      1126           

Flag	Coverage Δ
linux	98.93% <100.00%> (ø)
windows	100.00% <100.00%> (ø)

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Files with missing lines	Coverage Δ
...cstringformatter/_formatting/formatters_default.py	100.00% <100.00%> (ø)

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