Sort imports with isort

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.

tests/roots/test-ext-autodoc/target/__init__.py

@@ -3,7 +3,6 @@ from io import StringIO
 
 from sphinx.util import save_traceback  # NOQA
 
-
 __all__ = ['Class']
 
 #: documentation for the integer