Add extras_require for tests

.travis.yml

@@ -2,6 +2,6 @@ language: python
 python:
   - "3.6"
 install:
-  - pip install -e .
+  - pip install -e .[tests]
 script:
   - pytest

setup.py

@@ -13,9 +13,13 @@
     install_requires=[
         'pytest>=3.0,<4',
         'pydocstyle>=2.1,<2.2',
-        'pytest-codestyle>=1.0,<2.0',
-        'pytest-isort>=0.1,<1.0',
     ],
+    extras_require={
+        'tests': [
+            'pytest-codestyle>=1.0,<2.0',
+            'pytest-isort>=0.1,<1.0',
+        ],
+    },
     # https://docs.pytest.org/en/latest/writing_plugins.html#making-your-plugin-installable-by-others
     entry_points={
         'pytest11': [