add spopt ref to JOSS paper

[jGaboardi]

This PR adds the original spopt reference to JOSS submission.

[codecov]

Codecov Report

Merging #245 (1e71809) into main (886a9ac) will increase coverage by 0.2%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #245     +/-   ##
=======================================
+ Coverage   66.1%   66.3%   +0.2%     
=======================================
  Files         23      23             
  Lines       2582    2582             
  Branches     566     566             
=======================================
+ Hits        1706    1711      +5     
+ Misses       790     781      -9     
- Partials      86      90      +4     

Impacted Files	Coverage Δ
spopt/_version.py	33.8% <0.0%> (+1.5%)	⬆️