Change Makefile to use "python" rather than "python3"

[hyanwong]

Trivial I think, and matches the other tskit repos.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.85%. Comparing base (0909034) to head (db42724).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2369   +/-   ##
=======================================
  Coverage   90.85%   90.85%           
=======================================
  Files          20       20           
  Lines       11979    11979           
  Branches     2325     2325           
=======================================
  Hits        10883    10883           
  Misses        606      606           
  Partials      490      490           

Flag	Coverage Δ
C	90.85% <ø> (ø)
python	98.70% <ø> (ø)

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