Make failing a some flake8 tests cause the testsuite to fail. (Currently just "undefined name")

debian/control

@@ -21,6 +21,7 @@ Build-Depends:
  dpkg-dev (>= 1.17.14),
  device-tree-compiler (>= 1.4.2) <!nocheck>,
  enjarify <!nocheck>,
+ flake8 <!nocheck>,
  fontforge-extras <!nocheck>,
  fp-utils <!nocheck>,
  ghc <!nocheck>,

debian/rules

@@ -23,6 +23,7 @@ export PATH := $(CURDIR)/bin:$(PATH)
 export PYBUILD_DESTDIR = debian/diffoscope
 export PYBUILD_TEST_ARGS = -vv -r sxX -l --cov=diffoscope --cov-report=term-missing --cov-report=html
 export PYBUILD_AFTER_TEST = debian/tests/basic-command-line
+export PYBUILD_BEFORE_TEST = flake8
 export PYBUILD_AFTER_INSTALL = rm -rvf '{destdir}/{install_dir}/htmlcov' '{destdir}/{install_dir}/.coverage'
 
 ifneq ($(JENKINS_EXPORTS_DIR),)

setup.cfg

+[flake8]
+select=F821