Remove mock dependency

[rwedge]

This PR removes mock as a dependency to run the tests. Featuretools doesn't import mock directly. Another testing dependency, moto, also has mock as a dependency, so mock will still be installed in order to run the tests.

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After creating the pull request: in order to pass the changelog_updated check you will need to update the "Future Release" section of docs/source/changelog.rst to include this pull request.

[codecov]

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  Hits              ?    10049           
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