Updating TPL package list due to contrib.solver

[jsiirola]

Fixes # .

Summary/Motivation:

contrib.solver added an unconditional dependence on datetime. While we could defer that import, it is not a slow import (at least at the moment), and is likely to become pervasive throughout the solver interfaces, to this PR adds to the list of expected TPL imports.

This resolves an occasional test failure that checks the TPL import times.

Changes proposed in this PR:

• add datetime to the list of expected pyomo.environ TPL imports

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