utils: Move "header check" to a flake8 plugin

If we want to check style, we run 'tox -e flake8': it shouldn't be
necessary to run some obscure 'make' command too. Make this possible by
moving the sole useful test from the target of this make command to a
flake8 plugin.

This includes a fix for a header that was previously excluded from
checks, but is now included.

Signed-off-by: Stephen Finucane <stephen@that.guru>

setup.py

@@ -247,6 +247,13 @@ setup(
         'distutils.commands': [
             'build_sphinx = sphinx.setup_command:BuildDoc',
         ],
+        # consider moving this to 'flake8:local-plugins' once flake8 3.5.0 is
+        # in the wild:
+        #    http://flake8.pycqa.org/en/latest/user/configuration.html\
+        #    #using-local-plugins
+        'flake8.extension': [
+            'X101 = utils.checks:sphinx_has_header',
+        ],
     },
     install_requires=requires,
     extras_require=extras_require,