Investigate relying on tox's default install capabilities instead of an explicit command

From [ticket:26811#comment:2]:

I'm not convinced that the pip install -e . line needs to be in the tox.ini config //at all//, as I'm pretty sure tox will install things just fine without it, but I didn't have the time to explore that route further at the present.

So I think the best option moving forward in the interim is to take the patch, and file another ticket to look further into it at a later point. (But let me know your thoughts!)

This ticket is //that// ticket. Its purpose: investigate removing the pip install -e . line from the config (or note why it might be necessary).