Allow maingo_solvermodel to be imported without maingopy

[jsiirola]

Fixes # .

Summary/Motivation:

This resolves recent test failures in the GHA singletest job. MAiNGOpy (@MAiNGO-github) released a new version yesterday (8.1) that only included wheels for Windows. Unfortunately, the MAiNGOpy setup.py has an unconditional required import of skbuild (which is not present in the Pyomo GHA environment), so the build of maingopy fails on Linux & OSX. This highlighted that the pyomo.contrib.appsi.solvers.maingo_solvermodel module is not importable without maingopy (leading to the test failure).

This PR resolves that test failure (but not the availability of maingopy on GHA Linux / OSX builds).

Changes proposed in this PR:

• rework the imports of maingopy within the appsi_maingo solver so that module imports do not fail if maingopy is missing

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.50%. Comparing base (f86ac9d) to head (caa8cb5).
Report is 822 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3330      +/-   ##
==========================================
+ Coverage   88.30%   88.50%   +0.20%     
==========================================
  Files         868      868              
  Lines       98436    98418      -18     
==========================================
+ Hits        86928    87109     +181     
+ Misses      11508    11309     -199     

Flag	Coverage Δ
linux	86.02% <88.88%> (+0.26%)	⬆️
osx	75.29% <66.66%> (+<0.01%)	⬆️
other	86.52% <100.00%> (+0.22%)	⬆️
win	83.83% <100.00%> (+0.01%)	⬆️

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