Add a 'tox -e flake8'

setup.cfg

@@ -13,3 +13,20 @@ python = python
 
 [bdist_wheel]
 universal = 0
+
+[flake8]
+builtins = _
+max-line-length = 80
+extend-ignore =
+# https://pep8.readthedocs.org/en/latest/intro.html#error-codes
+# these are ignored by default:
+# E121: continuation line under-indented for hanging indent
+# E123: closing bracket does not match indentation of opening bracket’s line
+# E126: continuation line over-indented for hanging indent
+# E133: closing bracket does not match visual indentation
+# E226: missing whitespace around arithmetic operator
+# E241: multiple spaces after ‘,’
+# E242: tab after ‘,’
+# E704: multiple statements on one line (def)
+# W503: line break before binary operator
+# W504: line break after binary operator

tox.ini

@@ -31,3 +31,8 @@ deps =
     pyxdg
     dnspython
     {[base]deps}
+
+[testenv:flake8]
+deps = flake8
+skip_install = true
+commands = flake8 setup.py linkcheck {posargs}