Compatibility with the latest flake8 (#2373)

Require flake8 at least 3.0.0 and just call main()

setup.py

@@ -93,12 +93,10 @@ class Flake8(_Command):
         return [package for package in self.package_files()] + ['tests', 'setup.py']
 
     def run(self):
-        from flake8.main import application
+        from flake8.main.cli import main
 
         logging.getLogger().setLevel(logging.ERROR)
-        flake8 = application.Application()
-        flake8.run(self.targets())
-        flake8.exit()
+        main(self.targets())
 
 
 class PyTest(_Command):