Add mock package to RTD requirements

[jkarni]

@adampalay @singingwolfboy

[singingwolfboy]

Looks good to me. Python projects often have a requirements.txt for project dependencies, and a separate test-requirements.txt for dependencies used only for running tests -- but it looks like we don't have anything like that, so sticking mock in the general requirements is fine. 👍

[jkarni]

@singingwolfboy This is the requirements.txt for docs only; we already otherwise have mock. But for a couple of reasons, RTD can't use the same set of requirement files, and so we need this separate one.

[adampalay]

👍