deprecate: setEquation deprecation

[cadenmyers13]

Each deprecated function emits 1 deprecation warning for each test. Thus far, 4 functions deprecated = 4 deprecation warnings emitted

[cadenmyers13]

@sbillinge ready for review

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.82%. Comparing base (1e60597) to head (a6a7316).
⚠️ Report is 26 commits behind head on v3.3.0.

Additional details and impacted files

@@            Coverage Diff             @@
##           v3.3.0     #150      +/-   ##
==========================================
+ Coverage   69.36%   69.82%   +0.46%     
==========================================
  Files          25       25              
  Lines        3457     3510      +53     
==========================================
+ Hits         2398     2451      +53     
  Misses       1059     1059              

Files with missing lines	Coverage Δ
tests/conftest.py	91.48% <100.00%> (ø)
tests/test_contribution.py	99.55% <100.00%> (+0.04%)	⬆️
tests/test_fitrecipe.py	99.78% <100.00%> (+0.01%)	⬆️
tests/test_weakrefcallable.py	98.91% <100.00%> (ø)

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[sbillinge]

this looks good. Are there actually tests for the set_equation (and setEquation) methods? I couldn't see them.

[cadenmyers13]

@sbillinge yes, theres test_setEquation (old) and test_set_equation (new). test_setEquation was untouched and therefore not shown in this PR.