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-root/autodoc_target.py

@@ -1,7 +1,6 @@
 import enum
 from io import StringIO
 
-
 __all__ = ['Class']
 
 #: documentation for the integer

tests/roots/test-root/conf.py

@@ -6,7 +6,6 @@ from docutils.parsers.rst import Directive
 
 from sphinx import addnodes
 
-
 sys.path.append(os.path.abspath('.'))
 
 extensions = ['sphinx.ext.autodoc',