Update dev dependency due lack of python support 3.8 (flake8)

Author: pitbulk

setup.py

@@ -48,7 +48,7 @@
             'coverage>=4.5.2',
             'freezegun>=0.3.11, <=1.1.0',
             'pylint==1.9.4',
-            'flake8==3.6.0',
+            'flake8>=3.6.0',
             'coveralls==1.5.1',
         ),
     },