Fix flake8 errors and warnings

We add the flake8 configuration used so we ensure everyone uses the same.
We remove the usage of __all__ for non public API.
When possible, we rationalize the class name according to its __name__ and module.

issue9082
review297061002

tests/__init__.py

@@ -2,7 +2,7 @@
 # this repository contains the full copyright notices and license terms.
 
 try:
-    from trytond.modules.account_dunning_email.tests.test_account_dunning_email import suite
+    from trytond.modules.account_dunning_email.tests.test_account_dunning_email import suite  # noqa: E501
 except ImportError:
     from .test_account_dunning_email import suite