Add `unittest.mock` backport

[ruaridhw]

Fixes:

Summary/Motivation:

• To allow usage of unittest.mock in packages that import pyutilib.th as unittest

Changes proposed in this PR:

-

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[jsiirola]

It looks like the hard dependence on the mock library might be causing problems (I rarely use mock, and in our other discussion missed that mock was a separate package before Python 3.3). I am not sure how others will feel about adding another dependency. At a minimum, we should promulgate the mock from unittest in Python3:

if six.PY3:
    from unittest import mock
else:
    import mock

I wonder if it also makes sense to make the dependency optional:

if six.PY3:
    from unittest import mock
    mock_available = True
else:
    try:
        import mock
        mock_available = True
    except ImportError:
        mock_available = False

Thoughts?

[ruaridhw]

Updated accordingly though I figured that using Python3 doesn't necessarily imply that mock exists in unittest since it wasn't around in 3.0

[jsiirola]

@ruaridhw: correct, but PyUtilib explicitly doesn't support 3.0-3.3, so we can be lazy.

[coveralls]

Coverage decreased (-0.01%) to 61.919% when pulling ceada22944805c270f389d83f49f3e10aee67b8d on flexciton:feat/mock into 6f02a998a57f1e843dcde3409fad58cd15005c43 on PyUtilib:master.