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
@@ -27,4 +28,4 @@ deploy:
     secure: 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
   distributions: sdist bdist_wheel
   on:
-    tags: true
+    tags: true

requirements_dev.txt

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

setup.py

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