Update to flake8 3.8.x

In particular this improves support for detecting usage of various
type annotation usages and adds support for correctly parsing
type: ignore comments which contain a reason tag.

setup.cfg

@@ -13,6 +13,8 @@ ignore =
   E721,
   # Line break before binary operator
   W503,
+  # Single letter loop variables are often fine
+  E741,
 exclude = jedi/third_party/* .tox/*
 
 [pycodestyle]