[PATCH] tox: Specify doctree directory

[Stephen Finucane]

On Tue, 2018-09-18 at 16:49 +0100, Stephen Finucane wrote:
> Sphinx 1.8 has a change in where it places 'doctree' directories. Rather
> than ignore this directory via gitignore, specify where this directory
> should go. This will ensure future changes in Sphinx's behavior won't
> affect us.
> 
> Signed-off-by: Stephen Finucane <stephen at that.guru>

Applied as this is trivial and was annoying me.