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/test_quickstart.py

@@ -15,10 +15,9 @@ import pytest
 
 from sphinx import application
 from sphinx.cmd import quickstart as qs
-from sphinx.util.console import nocolor, coloron
+from sphinx.util.console import coloron, nocolor
 from sphinx.util.pycompat import execfile_
 
-
 warnfile = StringIO()