Fix some problems with ExtensibleRate

[speth]

Changes proposed in this pull request

• Fix de-registration of "change callbacks" from Python. I'm pretty sure this is the bug causing the failures noted in Occasional segfault in ExtensibleRate tests #1489.
• Fix working with locally-instantiated ExtensibleRate objects so they can also be retrieved from the Solution object

If applicable, fill in the issue number this pull request is fixing

Closes #1489

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

Merging #1494 (eef6f0c) into main (0d449ca) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1494   +/-   ##
=======================================
  Coverage   69.50%   69.50%           
=======================================
  Files         377      377           
  Lines       57993    57995    +2     
  Branches    20693    20693           
=======================================
+ Hits        40310    40312    +2     
  Misses      14732    14732           
  Partials     2951     2951           

Impacted Files	Coverage Δ
interfaces/cython/cantera/reaction.pyx	82.22% <100.00%> (+0.03%)	⬆️
interfaces/cython/cantera/solutionbase.pyx	89.37% <100.00%> (+0.03%)	⬆️

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[ischoegl]

Thanks, @speth! Looks good to me.