Since python3.5, subprocess.run() has been introduced. It works a
wrapper of Popen, and it looks much simple and better. This uses it
instead of Popen to make our code simple.

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>