add pytest as test requirement to setup.py

Author: Dominik Neise

setup.py

@@ -48,6 +48,7 @@
     },
 
     install_requires=['mpyq>=0.2.3', 'argparse', 'ordereddict', 'unittest2', 'pil'] if float(sys.version[:3]) < 2.7 else ['mpyq>=0.2.4'],
+    tests_require=["pytest"],
     packages=setuptools.find_packages(),
     include_package_data=True,
     zip_safe=True