Fixes # .

Summary/Motivation:

A new release of sphinx-jinja2-compat (0.4.0; used by sphinx-toolbox) now causes sphinx to be automatically imported any time Python is started (a rather evil practice...). This broke the current logic for detecting if Sphinx was running.

This PR changes the logic so that Pyomo's Sphinx conf.py sets a flag in the pyomo module namespace when it is run -- which building_documentation() can look for and report.

Changes proposed in this PR:

• Use an explicit flag (and not looking for loaded modules) to tell is Sphinx is running
• Remove dependency on sphinx-toolbox

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