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.

Signed-off-by: oleg.hoefling <oleg.hoefling@gmail.com>

Since 4.0, Sphinx does not use lib2to3.  Therefore a filterwarnings
entry for lib2to3 is no longer needed.