Upgrade isort to 4.2.2.

This gets rid of a file encoding bug that occurs on Windows boxes where
the default implied encoding is cp1252, but the files themselves aren't.

The actual patch that seems to fix this is
https://github.com/timothycrosley/isort/commit/1057d5e12edbb3030045b7a7a8c82f0848635fef
but it seems like a good idea to upgrade to the latest and greatest.

requirements/requirements-codestyle.txt

@@ -3,4 +3,4 @@ flake8==2.4.0
 pep8==1.5.7
 
 # Sort and lint imports
-isort==3.9.6
+isort==4.2.2