Correct output dimension for ts.pair_coalescence_counts with extra time windows

[nspope]

Fixes #3046

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.85%. Comparing base (9acedd2) to head (b99dfe7).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3058   +/-   ##
=======================================
  Coverage   89.85%   89.85%           
=======================================
  Files          29       29           
  Lines       32128    32137    +9     
  Branches     5763     5765    +2     
=======================================
+ Hits        28868    28877    +9     
  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/trees.py	98.80% <100.00%> (+<0.01%)	⬆️

... and 1 file with indirect coverage changes

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[nspope]

Shall we merge this, then I'll rebase #3059 and update the changelog?

[nspope]

mergify issues again:

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