Use the dependencies declaration to specify test dependencies

[pelson]

Use the dependencies declaration to specify test dependencies, rather than embedding them in the tox.ini

cc @agjohnson following on from #1345 (comment) (which I reverted in that PR)

[stsewd]

Just a note, some maintainers are packaging this theme for some distributions, and they include tests (so they also need to package the test dependencies), I wasn't able to find the conversation, but would be nice to not have maintainers package the readthedocs-sphinx-ext extension, since it's only used internally by RTD.

I think it probably makes sense to have those tests in https://github.com/readthedocs/readthedocs-sphinx-ext, or have them conditionally run or something.

[pelson]

I'll withdraw this one - it wasn't working with tox, and I don't know it well enough to figure out the magic incantation. Furthermore, it seems that the desired solution isn't to correctly document the dependency, but rather to remove that dependency from the project.