Fix requirements list for development and tests.

`tests_require` should list only libraries need for running *unit* tests.
Libraries and apps needed for linting/coverage/etc. should be listed
under the 'dev' installation mode.

Author: josemauro

setup.py

@@ -119,8 +119,9 @@ def run(self):
       test_suite='tests',
       include_package_data=True,
       setup_requires=['pytest-runner'],
-      tests_require=['coverage', 'pytest', 'yala', 'tox'],
-      extras_require={'dev': ['pip-tools >= 2.0']},
+      tests_require=['pytest'],
+      extras_require={'dev': ['pip-tools >= 2.0',
+                              'coverage', 'pytest', 'yala', 'tox']},
       packages=find_packages(exclude=['tests']),
       cmdclass={
           'ci': CITest,