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

.flake8

@@ -0,0 +1,2 @@
+[flake8]
+ignore=E123,E124,E126,E128,W503

account.py

@@ -12,9 +12,6 @@ from trytond.sendmail import sendmail_transactional, SMTPDataManager
 from trytond.transaction import Transaction
 from trytond.wizard import StateTransition
 
-__all__ = ['Configuration', 'DunningLevel', 'ProcessDunning', 'Dunning',
-    'DunningEmailLog']
-
 
 class Configuration(metaclass=PoolMeta):
     __name__ = 'account.configuration'

setup.py

@@ -79,8 +79,8 @@ setup(name=name,
     keywords='tryton account dunning email',
     package_dir={'trytond.modules.account_dunning_email': '.'},
     packages=(
-        ['trytond.modules.account_dunning_email'] +
-        ['trytond.modules.account_dunning_email.%s' % p
+        ['trytond.modules.account_dunning_email']
+        + ['trytond.modules.account_dunning_email.%s' % p
             for p in find_packages()]
         ),
     package_data={
@@ -95,7 +95,8 @@ setup(name=name,
         'Intended Audience :: Developers',
         'Intended Audience :: Financial and Insurance Industry',
         'Intended Audience :: Legal Industry',
-        'License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)',
+        'License :: OSI Approved :: '
+        'GNU General Public License v3 or later (GPLv3+)',
         'Natural Language :: Bulgarian',
         'Natural Language :: Catalan',
         'Natural Language :: Czech',

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