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.
Trailing whitespace do not have impact on the result, they are just
unused bytes. Most text editors are configured to strip trailing
whitespaces. Remove them all in one go.
Update a handful of files to use the UNIX line ending.
