Scheduled monthly dependency update for September (#256)

* Update sphinx from 1.7.6 to 1.7.8

* Update attrs from 18.1.0 to 18.2.0

* Update pytest from 3.7.2 to 3.7.4

docs/requirements.txt

@@ -1,4 +1,4 @@
 docutils==0.14
 Pygments==2.2.0
-Sphinx==1.7.6
+Sphinx==1.7.8
 sphinxcontrib-websupport==1.1.0

tests/requirements.txt

@@ -1,11 +1,11 @@
-attrs==18.1.0
+attrs==18.2.0
 coverage==4.5.1
 flake8==3.5.0
 isort==4.3.4
 mypy==0.620
 pycodestyle==2.3.1  # pyup: ignore
 pyflakes==2.0.0
-pytest==3.7.2
+pytest==3.7.4
 pytest-cov==2.5.1
 pytest-isort==0.2.0
 pytest-mock==1.10.0