Run flake8 tests as part of test suite.

.flake8

+[flake8]
+exclude=
+    .tox
+    setuptools/_vendor,
+    pkg_resources/_vendor
+ignore =
+    # W503 violates spec https://github.com/PyCQA/pycodestyle/issues/513
+    W503
+    # W504 has issues https://github.com/OCA/maintainer-quality-tools/issues/545
+    W504
+    setuptools/site-patch.py F821
+    setuptools/py*compat.py F811

pytest.ini

 [pytest]
-addopts=--doctest-modules --doctest-glob=pkg_resources/api_tests.txt -r sxX
-norecursedirs=dist build *.egg setuptools/extern pkg_resources/extern pkg_resources/tests/data tools .*
-flake8-ignore =
-    setuptools/site-patch.py F821
-    setuptools/py*compat.py F811
+addopts=--doctest-modules --flake8 --doctest-glob=pkg_resources/api_tests.txt -r sxX
+norecursedirs=dist build *.egg setuptools/extern pkg_resources/extern pkg_resources/tests/data tools .* setuptools/_vendor pkg_resources/_vendor
 doctest_optionflags=ELLIPSIS ALLOW_UNICODE
 filterwarnings =
 	# https://github.com/pypa/setuptools/issues/1823