Drop mock dependency

[jelly]

Part of the tests already use unittest.mock so switch the last mock module user over and drop the mock test dependency.

Description (e.g. "Related to ...", etc.)

Drop mock dependency

Code review checklist (for code reviewer to complete)

• [x] Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• [x] Title summarizes what is changing
• [x] Commit messages are meaningful (see this for details)
• [x] Tests have been included and/or updated, as appropriate
• [/] Docstrings have been included and/or updated, as appropriate
• [/] Standalone docs have been updated accordingly
• [ ] CONTRIBUTORS.md was updated, as appropriate
• [/] Changelog has been updated, as needed (see CHANGELOG.md)

[tombh]

Thank you! (And thanks for all your Arch work too)

Would you like to add yourself to CONTRIBUTORS.md?

[jelly]

Thank you! (And thanks for all your Arch work too)

Thanks!

Would you like to add yourself to CONTRIBUTORS.md?

Done :)