Use unittest.mock if it is available

[major]

The python mock module is deprecated in recent Python versions. If unittest.mock is available in the system's python, use that instead.

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Fixes#207 🦕

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[major]

@googlebot I signed it!

[tseaver]

@major Thanks for the patch! Rather than add the unittest.mock import conditionally, I think we would prefer to fold in making it unconditional at the same time we drop support for Python 2.7.

[major]

@tseaver That's fine -- just thought I'd offer to help! Feel free to close this if you want.

[tseaver]

Superseded by #210.