Keep imports alphabetically sorted and their order homogeneous across
Python source files.

The isort project has more feature and is more active than the
flake8-import-order plugin.

Most issues caught were simply import ordering from the same module.
Where imports were purposefully placed out of order, tag with
isort:skip.

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>

Fixes #4035