Make type checking pass with TY

[jenshnielsen]

Mainly me experimenting with how ready TY is.

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.10%. Comparing base (988327e) to head (e319f5b).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/broadbean/_version.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1653      +/-   ##
==========================================
- Coverage   72.12%   72.10%   -0.02%     
==========================================
  Files           9        9              
  Lines        1539     1538       -1     
==========================================
- Hits         1110     1109       -1     
  Misses        429      429              

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