BLD/RFC: use Cython templating instead of compile-time conditionals

[neutrinoceros]

Alternative to #1574

• Closes Cython deprecating compile time conditional statements #1142
• [N/A] Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.85%. Comparing base (e81e5e5) to head (d292bec).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1577   +/-   ##
=======================================
  Coverage   95.85%   95.85%           
=======================================
  Files          20       20           
  Lines        1880     1880           
=======================================
  Hits         1802     1802           
  Misses         78       78           

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[neutrinoceros]

(rebased to resolve a trivial merge conflict)