Adding a compatibility shim for assertRaisesRegex

[jsiirola]

Fixes: Pyomo/pyomo#1283

Summary/Motivation:

assertRaisesRegexp was added in Python 3.2 and now issues a deprecation warning beginning in Python 3.8. This adds a definition of assertRaisesRegex to the pyutilib unittest wrapper in Python 2.

Changes proposed in this PR:

• define assertRaisesRegex in Python 2.

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[coveralls]

Coverage decreased (-0.01%) to 61.039% when pulling da2f6ae on unittest-compat into 5964c9c on master.

[codecov-io]

Codecov Report

Merging #79 into master will increase coverage by 0.17%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage    62.9%   63.07%   +0.17%     
==========================================
  Files          87       87              
  Lines        8807     8783      -24     
==========================================
  Hits         5540     5540              
+ Misses       3267     3243      -24

Impacted Files	Coverage Δ
pyutilib/th/pyunit.py	47.13% <0%> (-0.32%)	⬇️

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