Fix br tags in output

[benjeffery]

Fixes #3039

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.85%. Comparing base (eb2292a) to head (a807f87).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3040   +/-   ##
=======================================
  Coverage   89.85%   89.85%           
=======================================
  Files          29       29           
  Lines       32127    32128    +1     
  Branches     5763     5763           
=======================================
+ Hits        28867    28868    +1     
  Misses       1859     1859           
  Partials     1401     1401           

Flag	Coverage Δ
c-tests	86.69% <ø> (ø)
lwt-tests	80.78% <ø> (ø)
python-c-tests	89.05% <ø> (ø)
python-tests	98.98% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
python/tskit/util.py	99.25% <100.00%> (+<0.01%)	⬆️