tox: add flake8 for style and other checking

setup.cfg

@@ -64,8 +64,21 @@ source = src
 [mypy]
 ignore_missing_imports = true
 
+
+[flake8]
+# please note that the values are adjusted so that they do not cause failures
+# with existing code. if you want to change them, you should first fix all
+# flake8 failures that appear with your change.
+ignore =
+    W503,
+    F401,F821,F841,
+    E712
+# line length long term target: 120
+max-line-length = 150
+exclude = build,dist,.git,.idea,.cache,.tox
+
 [tox:tox]
-envlist = py36
+envlist = py36,flake8
 
 [testenv]
 deps =
@@ -74,3 +87,8 @@ deps =
     pytest-mock
     pytest-faulthandler
 commands=pytest
+
+[testenv:flake8]
+deps =
+    flake8
+commands=flake8 src tests