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

setup.py

@@ -79,8 +79,9 @@ setup(name=name,
     keywords='tryton stock secondary unit',
     package_dir={'trytond.modules.stock_secondary_unit': '.'},
     packages=(
-        ['trytond.modules.stock_secondary_unit'] +
-        ['trytond.modules.stock_secondary_unit.%s' % p for p in find_packages()]
+        ['trytond.modules.stock_secondary_unit']
+        + ['trytond.modules.stock_secondary_unit.%s' % p
+            for p in find_packages()]
         ),
     package_data={
         'trytond.modules.stock_secondary_unit': (info.get('xml', [])
@@ -94,7 +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 :: Chinese (Simplified)',

tests/__init__.py

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