Remove future imports

There were two used:

- print_function
- absolute_import

Both of these are mandatory in Python 3.0 onwards [1] and can therefore
be removed...mostly. Unfortunately, mypy is still running in Python 2.7
mode, meaning we need the 'print_function' future wherever we're calling
'print' with the 'file' argument. There's also a single
'absolute_import' future that must be retained as its removal breaks a
test for as-yet unknown reasons.  TODOs are added to resolve both issues
in the future.

[1] https://docs.python.org/3/library/__future__.html

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.

The form is less verbose and more idiomatic for Python 3 only code.

https://docs.python.org/3/library/functions.html#super