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.
In Python 3, the default encoding of source files is utf-8. The encoding
cookie is now unnecessary and redundant so remove it. For more details,
see the docs:
https://docs.python.org/3/howto/unicode.html#the-string-type
> The default encoding for Python source code is UTF-8, so you can
> simply include a Unicode character in a string literal ...
Includes a fix for the flake8 header checks to stop expecting an
encoding cookie.
