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

@@ -6,7 +6,6 @@ from trytond.transaction import Transaction
 from trytond.pyson import Eval
 from trytond.modules.company.model import CompanyValueMixin
 
-__all__ = ['Configuration', 'ConfigurationDefaultCreditLimitAmount', 'Level']
 default_credit_limit_amount = fields.Numeric(
     "Default Credit Limit Amount",
     help="The default credit limit amount for new customers.",

party.py

@@ -11,8 +11,6 @@ from trytond.modules.company.model import CompanyValueMixin
 
 from .exceptions import CreditLimitError, CreditLimitWarning
 
-__all__ = ['Party', 'PartyCreditLimitAmount']
-
 
 class Party(metaclass=PoolMeta):
     __name__ = 'party.party'

setup.py

@@ -79,8 +79,8 @@ setup(name=name,
     keywords='tryton account credit limit',
     package_dir={'trytond.modules.account_credit_limit': '.'},
     packages=(
-        ['trytond.modules.account_credit_limit'] +
-        ['trytond.modules.account_credit_limit.%s' % p
+        ['trytond.modules.account_credit_limit']
+        + ['trytond.modules.account_credit_limit.%s' % p
             for p in find_packages()]
         ),
     package_data={
@@ -94,7 +94,8 @@ 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 :: Chinese (Simplified)',

tests/__init__.py

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