Always use unittest.mock rather than PyPI mock.

[musicinmybrain]

Since only Python 3.6+ is supported, there is no need to use the mock backport package. This commit ensures that only the standard library’s unittest.mock is needed.

Replaces a mixture of conditional and unconditional imports of mock and/or unittest.mock with unconditional imports of unittest.mock, and drops mock from requirements.txt.

[ghost]

All CLA requirements met.

[jiasli]

Thanks for the contribution! Nice change!