Remove future imports

If Sphinx is Python 3-only, Sphinx's tooling should be Python 3-only
too.

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

In Python 3, the default encoding of source files is utf-8. The encoding
cookie is now unnecessary and redundant so remove it. For more details,
see the docs:

https://docs.python.org/3/howto/unicode.html#the-string-type

> The default encoding for Python source code is UTF-8, so you can
> simply include a Unicode character in a string literal ...

Includes a fix for the flake8 header checks to stop expecting an
encoding cookie.

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>