Restore compatibility with expected minimum Cython version

[speth]

Changes proposed in this pull request

The stated minimum version of Cython is 0.29.31, in both the documentation of the dependencies and as checked by SCons. However, Cantera currently fails to compile except with Cython 3.0.0 and newer due to differences in the way __add__/__radd__ and related operators are implemented.

While I expect we'll probably drop support for anything older than Cython 3.0.0 after the Cantera 3.1 release, I'd like to maintain support for these older versions for the time being.

This also updates the CI configuration to make sure we're testing more than just the latest and greatest Cython version.

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

Attention: Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.

Project coverage is 73.23%. Comparing base (4a954f9) to head (6320dd4).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
interfaces/cython/cantera/func1.pyx	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1798      +/-   ##
==========================================
- Coverage   73.24%   73.23%   -0.01%     
==========================================
  Files         381      381              
  Lines       54377    54390      +13     
  Branches     9066     9066              
==========================================
+ Hits        39826    39833       +7     
- Misses      11580    11585       +5     
- Partials     2971     2972       +1     

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[ischoegl]

@speth ... thanks for catching this. I was honestly under the impression that we did test multiple Cython versions.