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.

doc/development/tutorials/examples/recipe.py

@@ -4,8 +4,7 @@ from docutils.parsers.rst import directives
 
 from sphinx import addnodes
 from sphinx.directives import ObjectDescription
-from sphinx.domains import Domain
-from sphinx.domains import Index
+from sphinx.domains import Domain, Index
 from sphinx.roles import XRefRole
 from sphinx.util.nodes import make_refnode