Replace PyPI mock test dependency with unittest.mock

[musicinmybrain]

The unittest.mock module is part of the standard library since Python 3.3, so there is no longer any benefit to carrying the PyPI package as a test dependency.

See https://fedoraproject.org/wiki/Changes/DeprecatePythonMock for further context.

[codecov-commenter]

Codecov Report

Merging #1084 (13bf54b) into master (43005b3) will not change coverage. The diff coverage is n/a.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1084 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 36 36 Lines 2452 2452 ========================================= Hits 2452 2452 ```

[musicinmybrain]

Thanks!