add Python 3.7 to setup.py

Author: raphaelhuefner

setup.py

@@ -31,11 +31,13 @@
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3 :: Only',
         'Natural Language :: English',
     ],
     keywords='gitcoin api client bounties bounty rest',
     packages=find_packages(exclude=['docs', 'tests']),
+    python_requires='~=3.4',
     install_requires=['requests'],
     setup_requires=['pytest-runner'],
     tests_require=['pytest', 'pytest-isort', 'pytest-cov', 'coverage', 'isort', 'responses'],