add 3.7 to supported version list

Author: wwwjfy

setup.py

@@ -58,6 +58,7 @@ def req_file(filename):
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
     ],
     project_urls={
         'Documentation': 'https://python-gino.readthedocs.io/',