docs: use picky mode and fix docs/helpers.rst

> docs/helpers.rst::py:class reference target not found: string

Author: blueyed

docs/helpers.rst

@@ -65,7 +65,7 @@ when trying to access the database.
 
    Specify a different ``settings.ROOT_URLCONF`` module for the marked tests.
 
-   :type urls: string
+   :type urls: str
    :param urls:
      The urlconf module to use for the test, e.g. ``myapp.test_urls``.  This is
      similar to Django's ``TestCase.urls`` attribute.

tox.ini

@@ -62,7 +62,7 @@ commands =
 [testenv:docs]
 deps =
 extras = docs
-commands = sphinx-build -W -b html -d docs/_build/doctrees docs docs/_build/html
+commands = sphinx-build -n -W -b html -d docs/_build/doctrees docs docs/_build/html
 
 [testenv:readme]
 basepython = python3.5