tox: fix docs target

I had to add atomicwrites to deps to fix:

    Warning, treated as error:
    autodoc: failed to import class 'Mark' from module
    '_pytest.mark.structures'; the following exception was raised:
    No module named 'atomicwrites'
    ERROR: InvocationError for command
    '/home/user/Vcs/pytest/.tox/docs/bin/sphinx-build -W -b html . _build'
    (exited with code 2)

I do not know why this is not failing on Travis, where atomicwrites gets
installed for some reason [1].
Removing `usedevelop = True` also fixes it, apparently since then
`install_requires` from `setup.py` gets used.

1: https://travis-ci.org/pytest-dev/pytest/jobs/388958175#L470

Author: blueyed

tox.ini

@@ -111,11 +111,8 @@ deps = {[testenv:py27-pluggymaster]deps}
 
 [testenv:docs]
 skipsdist = True
-usedevelop = True
 changedir = doc/en
 deps =
-    attrs
-    more-itertools
     PyYAML
     sphinx
     sphinxcontrib-trio