Simplify Python extension loading and other Python improvements

[speth]

Changes proposed in this pull request

• Simplify loading of Python extension support by letting Cython handle the complexity of loading the extension module after Python has already been initialized. In the case of a non-Python main program, all we have to do is initialize Python and import the Cantera Python module.
• Add support for Cython 3.0.0b1
• Convert C++ NotImplementedError to Python native NotImplementedError
• Improve documentation of _SolutionBase._references

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 #1456 (e2d6d23) into main (0c5fc1d) will increase coverage by 0.06%.
The diff coverage is 90.38%.

@@            Coverage Diff             @@
##             main    #1456      +/-   ##
==========================================
+ Coverage   69.86%   69.92%   +0.06%     
==========================================
  Files         373      377       +4     
  Lines       56684    57262     +578     
  Branches    18883    19151     +268     
==========================================
+ Hits        39600    40043     +443     
- Misses      14571    14669      +98     
- Partials     2513     2550      +37     

Impacted Files	Coverage Δ
interfaces/cython/cantera/mixture.pyx	90.47% <ø> (ø)
interfaces/cython/cantera/reactor.pyx	90.54% <ø> (ø)
interfaces/cython/cantera/solutionbase.pyx	89.66% <ø> (ø)
include/cantera/cython/funcWrapper.h	70.83% <50.00%> (-0.36%)	⬇️
interfaces/cython/cantera/delegator.pyx	79.91% <89.74%> (+1.09%)	⬆️
interfaces/cython/cantera/_onedim.pyx	79.76% <100.00%> (+0.27%)	⬆️
interfaces/cython/cantera/composite.py	89.16% <100.00%> (+0.09%)	⬆️
interfaces/cython/cantera/func1.pyx	100.00% <100.00%> (ø)
interfaces/cython/cantera/thermo.pyx	92.35% <100.00%> (ø)

... and 26 files with indirect coverage changes

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[ischoegl]

@speth ... this looks great overall. The only item I'd request would be a todo comment to move sentinels to C++ after Cantera 3.0.

[speth]

I created an issue (#1457) to track the needed feature. I think that's preferable to a to-do item buried somewhere in the code.

[ischoegl]

I created an issue (#1457) to track the needed feature. I think that's preferable to a to-do item buried somewhere in the code.

Thanks for creating the issue! I agree that it is preferable to a todo.

It did, however, occur to me that SolutionArray needs a similar sentinel, somewhere around here:

cantera/interfaces/cython/cantera/composite.py

Lines 558 to 559 in fbfd93f

	def __init__(self, phase, shape=(0,), states=None, extra=None, meta={}, init=True):
	self._phase = phase

Even prior to the introduction of C++ SolutionArray, adding species would have created inconsistencies in stored data.

PS: Another item I noticed is that we may want to test with Cython 3.0 again?

[speth]

I've added the Python weakref-style checks to both SolutionArray and Quantity.

I've also added a new CI job that runs only after pushes to main, and tests whatever the current pre-release version of Cython is. This creates a home for some of the tests that we've discussed in Cantera/enhancements#37, where running them on every push to a PR is more trouble than it's worth. I tested it by pushing this branch directly to Cantera/cantera. You can see the status (green checkmark) for this job reported here: https://github.com/Cantera/cantera/tree/python-ext-init. I'll delete that branch after we merge this PR.

[ischoegl]

Thanks, @speth … everything looks good to me!