Switch test requirement to pytest-flake8.

Flake8 runs both pyflakes and codestyle. Pep8 was renamed to
and supported as codestyle.

.travis.yml

@@ -13,6 +13,7 @@ matrix:
 before_install:
 - if [[ $TRAVIS_PYTHON_VERSION == '3.2' ]]; then pip install 'coverage<4.0.0'; fi
 - pip install codecov
+- pip install 'py>=1.5.0'
 install:
 - pip install -e .
 - pip install -r requirements_dev.txt

requirements_dev.txt

@@ -1,6 +1,5 @@
 mock
 pytest
-pytest-pep8
+pytest-flake8
-pytest-flakes
 pytest-cov
 flask

setup.py

@@ -38,8 +38,7 @@ class PyTest(TestCommand):
         TestCommand.finalize_options(self)
         self.test_args = [
             '-sv',
-            '--pep8',
+            '--flake8',
-            '--flakes',
             '--junitxml', 'reports/junit.xml',
             '--cov', 'openapi_core',
             '--cov-report', 'term-missing',