Fix for PR #53 so yaml tests run with pyyaml<5.1

[jsiirola]

Fixes: N/A

Summary/Motivation:

PR #53 resolved deprecation warnings with recent PyYAML, but broke tests for PyYAML<5.1. This PR resolves those test failures so that tests run cleanly for new and old versions od PyYAML.

Changes proposed in this PR:

• Add a compatibility layer for supporting both PyYAML 5.1.x and 3.x.

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[coveralls]

Coverage remained the same at 62.245% when pulling 4cdaf4a on fix-yaml-tests into 7223482 on master.

[coveralls]

Coverage remained the same at 62.245% when pulling 4cdaf4a on fix-yaml-tests into 7223482 on master.

[coveralls]

Coverage remained the same at 62.245% when pulling 4cdaf4a on fix-yaml-tests into 7223482 on master.

[whart222]

The PR doesn't motivate the need for this fix. In general, we are not going to be able to support backwards compatibility in all cases for third-party libraries. So what's the motivation here? I know that this impacts Pyomo development, and I expect similar backwards compatibility issues to arise there as well.