Unfreezing mock dependency

In setup.py, I see that `mock` was frozen for a specific reason:

```
# TODO: Sync submodule with tag referenced here once #339 is fixed in mock.
'mock==2.0.0',
```

However, I see that https://github.com/testing-cabal/mock/issues/339 was fixed a while ago, and the upstream project has made [6 new releases](https://pypi.org/project/mock/#history) in May.

Can the mock dependency be unfrozen to support newer mock versions?

Start in setup.py at the pinned 'mock==2.0.0' dependency and review the linked upstream issue #339 and newer mock releases. Update the dependency constraint only if the upstream fix applies, then verify that the project still installs and its existing checks pass.

Written by the indexing model from the issue text.