Additional test coverage, and upgrade test requirements.

test_requirements.txt

@@ -1,25 +1,24 @@
 appdirs==1.4.3
-atomicwrites==1.2.1
-attrs==18.2.0
-black==18.9b0; python_version >= '3.6'
-Click==7.0
+atomicwrites==1.3.0
+attrs==19.1.0
+black==19.3b0; python_version >= '3.6'
+click==7.0
 colorama==0.4.1
-coverage==4.5.1
+coverage==4.5.3
 docopt==0.6.2
-flake8==3.5.0
+flake8==3.7.7
 flexmock==0.10.4
-isort==4.3.19
+isort==4.3.20
 mccabe==0.6.1
-more-itertools==4.3.0
-pluggy==0.7.1
-py==1.6.0
-pycodestyle==2.3.1
-pyflakes==2.0.0
+more-itertools==7.0.0
+pluggy==0.12.0
+py==1.8.0
+pycodestyle==2.5.0
+pyflakes==2.1.1
 pykwalify==1.7.0
-pytest==3.8.2
-pytest-cov==2.6.0
-python-dateutil==2.7.3
-PyYAML==3.13
+pytest==4.6.3
+pytest-cov==2.7.1
+python-dateutil==2.8.0
+PyYAML==5.1.1
 ruamel.yaml>0.15.0,<0.16.0
-six==1.11.0
 toml==0.10.0