Improve Tox test harness

- No need for both pycodestyle and flake8; flake8 includes the former.
- Use the proper pytest invocation.

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py34,py35,py36,pycodestyle,isort-check
+envlist = py27,py34,py35,py36,style,isort-check
 
 [testenv]
 deps =
@@ -10,22 +10,17 @@ deps =
     coverage
     taxii2-client
 commands =
-    py.test --ignore=stix2/test/test_workbench.py --cov=stix2 stix2/test/ --cov-report term-missing
+    pytest --ignore=stix2/test/test_workbench.py --cov=stix2 stix2/test/ --cov-report term-missing
-    py.test stix2/test/test_workbench.py --cov=stix2 --cov-report term-missing --cov-append
+    pytest stix2/test/test_workbench.py --cov=stix2 --cov-report term-missing --cov-append
 
 passenv = CI TRAVIS TRAVIS_*
 
-[testenv:pycodestyle]
+[testenv:style]
 deps =
   flake8
-  pycodestyle
 commands =
-  pycodestyle ./stix2
   flake8
 
-[pycodestyle]
-max-line-length=160
-
 [flake8]
 max-line-length=160
 
@@ -37,7 +32,7 @@ commands =
 
 [travis]
 python =
-  2.7: py27, pycodestyle
+  2.7: py27, style
-  3.4: py34, pycodestyle
+  3.4: py34, style
-  3.5: py35, pycodestyle
+  3.5: py35, style
-  3.6: py36, pycodestyle
+  3.6: py36, style