Reorder definitions to avoid NameError in some situations

[jsiirola]

Fixes #3262

Summary/Motivation:

This reorders some definitions to avoid a NameError reported by users.

Changes proposed in this PR:

• reorder imports / definitions
• remove compatibility code needed for Python < 3.8 (no longer supported by Pyomo)
• [unrelated] resolve deprecated use of pyplot.cm.get_cmap() in community_detection (deprecation path removed in matplotlib 3.9.0)

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[mrmundt]

Assuming tests pass, approved. Removes more code than it adds.