Fix unit tests on Python 2.6.
Review Request #9519 — Created Jan. 22, 2018 and submitted
The test suite was making use of testing functionality present only on
Python 2.7 and higher, breaking unit tests on older versions of Python.
To fix this, we now make use of the
unittest2package, which provides
backports of the testing functionality. This is only installed on older
versions of Python.
Unit tests pass on Python 2.6, 2.7, 3.4, 3.5, and 3.6.