Use mock from the stdlib.

[clokep]

Per @anoadragon453's suggestion from looking at #7704 this switches from using the mock package to the one in the standard library.

Done with some sed magic + isort.

• Switches from importing mock, to importing it as a submodule of unittest.
• Stop installing the mock package.

This affects test only code so should be fine if unit tests pass.

[clokep]

Looks like we use a few things on Mock that is only available in require Python >= 3.6. 😢 That's quite frustrating.

[clokep]

Unfortunately I think the best thing to do here is close this for now, we can re-open it once Python 3.5 support is dropped.