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>
