Update: unittest extension for python 3.x

[AIweimd]

Hi,
The mock object library has been integrated into the unittest module in python3.x, so I try to add extention like this.

try:

import mock  # python 2.x

except ModuleNotFoundError:

from unittest import mock  # python 3.x

Thanks for watching.

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[dbieber]

Thanks for the PR!

If I understand correctly, mock as a package from pypi continues to work in Python 3, and this change just allows us to drop the dependency on mock for testing purposes for Python 3.

Can you confirm I'm understanding correctly?

Could you also sign the CLA, please?