Try to fix pytest error on py27.

Author: yxd-hde

setup.py

@@ -45,6 +45,7 @@ def run_tests(self):
       packages=find_packages(exclude=['examples', 'tests']),
       include_package_data=True,
       zip_safe=False,
+      python_requires='>=2.7 >=3.6',
       tests_require=['pytest'],
       cmdclass={'test': PyTest},
       install_requires=['boto3'],