drop dependency on old external mock module

[a-detiste]

Hi,

As only Py3.8+ is supported, usage of old external mock does not makes sense anymore (It is stuck forever at the snapshot of stdlib from Py3.6) #345

Greetings

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https://github.com/testing-cabal/mock

mock is now part of the Python standard library, available as unittest.mock in Python 3.3 onwards.

[coveralls]

coverage: 76.752%. remained the same
when pulling aee25d6 on a-detiste:master
into bf01aac on diyan:master.

[jborean93]

Thanks for simplifying the test requirements.