feat: support python 3.8

Author: abawchen

requirements.txt

@@ -1,5 +1,5 @@
 coveralls==1.2.0
-flake8==3.5.0
+flake8==3.7.9
 flake8-per-file-ignores==0.6
 future==0.17.1
 graphene>=2.1.3,<3

setup.py

@@ -17,6 +17,7 @@
         "Programming Language :: Python :: 3.4",
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: Implementation :: PyPy",
         "License :: OSI Approved :: MIT License",
     ],