Used isort to order all import statements

doc/development/setup.rst

@@ -77,6 +77,7 @@ Code checks and unit tests
 Before you check in your code into git, always run the static checkers and unit tests::
 
     flake8 .
+    isort -c -rc .
     python manage.py validate
     py.test
 

doc/development/style.rst

@@ -22,6 +22,8 @@ Python code
  So please make sure that you *always* follow all other rules and break these rules *only when
  it makes sense*.
 
+* Use ``isort -rc pretix`` in the source directory to order your imports.
+
 * Indent your code with four spaces.
 
 * For templates and models, follow the `Django Coding Style`_.