Scheduled monthly dependency update for April (#449)

* Update sphinx from 1.8.4 to 2.0.0

* Update attrs from 18.2.0 to 19.1.0

* Update black from 18.9b0 to 19.3b0

* Update coverage from 4.5.2 to 4.5.3

* Update isort from 4.3.9 to 4.3.16

* Update pytest from 4.3.0 to 4.4.0

* Update pytest-isort from 0.2.1 to 0.3.1

* Update pytest-mock from 1.10.1 to 1.10.3

docs/requirements.txt

@@ -1,4 +1,4 @@
 docutils==0.14
 Pygments==2.3.1
-Sphinx==1.8.4
+Sphinx==2.0.0
 sphinxcontrib-websupport==1.1.0

tests/requirements.txt

@@ -1,13 +1,13 @@
-attrs==18.2.0
-black==18.9b0
-coverage==4.5.2
+attrs==19.1.0
+black==19.3b0
+coverage==4.5.3
 flake8==3.7.7
-isort==4.3.9
+isort==4.3.16
 mypy==0.670
 pycodestyle==2.5.0
 pyflakes==2.1.1
-pytest==4.3.0
+pytest==4.4.0
 pytest-cov==2.6.1
-pytest-isort==0.2.1
-pytest-mock==1.10.1
+pytest-isort==0.3.1
+pytest-mock==1.10.3
 pytest-sugar==0.9.2