refactor: use unittest.mock instead of mock

[himkt]

Since mock is ported to Python as unittest.mock.

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

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

[himkt]

@hainm I merged the master branch, I want you to review this PR as well.

[hainm]

Merged. Thanks @himkt.