Use tskit C API 1.2.0

[benjeffery]

Fixes #2423

[benjeffery]

Woop, lots of breakage. Will get to fixing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.23%. Comparing base (1df5329) to head (6d7e1be).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2424   +/-   ##
=======================================
  Coverage   91.23%   91.23%           
=======================================
  Files          20       20           
  Lines       11831    11831           
  Branches     2296     2296           
=======================================
  Hits        10794    10794           
  Misses        568      568           
  Partials      469      469           

Flag	Coverage Δ
C	91.23% <ø> (ø)
python	98.68% <ø> (ø)

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[benjeffery]

All fixed up!