Resolve incompatibility between imp and the DeferredImportCallbackFinder

[jsiirola]

Fixes # .

Summary/Motivation:

IDAES identified an incompatibility with Pyomo's DeferredImportCallbackFinder and the (deprecated) imp module. Finders from that module lack a find_spec method and instead use a find_module method.

This came up because Google Collab defines imp-style finders, which was causing errors: https://ndcbe.github.io/optimization/notebooks/7/degeneracy_hunter.html

Changes proposed in this PR:

• Support deferring to both imp and importlib style finders
• Add testing for Pyomo's callback hooks

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[lbianchi-lbl]

I tested the notebook (static link, Colab link) where the issue was originally noticed, making sure a new "runtime" (container instance) was used each time.

Without the fix (Pyomo 6.8.2)

With the fix (installing Pyomo from this PR's head branch before importing it)