Remove external mock dependency

[s-t-e-v-e-n-k]

Python 3.4 added mock to the unittest module, and we support a much newer version than that, so drop the requirement.

Checklist

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• SDK Code Change
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1 out of 2 committers have signed the CLA.

✅ AndreyVMarkelov
❌ s-t-e-v-e-n-k
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[ineil-dbx]

Hi @s-t-e-v-e-n-k , Thanks for submitting the pull request! I've shared it with our engineering team for review. We'll let you know once we have an update.