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.
Old SVG image contains non standard tags and attributes. Therefore,
epubcheck warns it as invalid. To avoid the warnings, this replaces it
by very simple one.
