Do extra code style checks with flake8-bugbear

Flake8 Bugbear checks for some extra things that aren’t code style
errors, but are likely to introduce bugs or unexpected behaviour. A
good example is having mutable default function arguments, which get
shared between every call to the function and therefore mutating a value
in one place can unexpectedly cause it to change in another.

This commit enables all the extra warnings provided by Flake8 Bugbear,
except for:
- the line length one (because we already lint for that separately)
- B903 Data class should either be immutable or use `__slots__` because
  this seems to false-positive on some of our custom exceptions
- B902 Invalid first argument 'cls' used for instance method because
  some SQLAlchemy decorators (eg `declared_attr`) make things that
  aren’t formally class methods take a class not an instance as their
  first argument

It disables:
- _B306: BaseException.message is removed in Python 3_ because I think
  our exceptions have a custom structure that means the `.message`
  attribute is still present

Matches the work done in other repos:
- https://github.com/alphagov/notifications-admin/pull/3172/files

tests/app/v2/notifications/test_post_letter_notifications.py

@@ -463,14 +463,24 @@ def test_returns_a_429_limit_exceeded_if_rate_limit_exceeded(
     assert not persist_mock.called
 
 
-@pytest.mark.parametrize('service_args', [
-    {'service_permissions': [EMAIL_TYPE, SMS_TYPE]},
-    {'restricted': True}
+@pytest.mark.parametrize('service_args, expected_status, expected_message', [
+    (
+        {'service_permissions': [EMAIL_TYPE, SMS_TYPE]},
+        400,
+        'Service is not allowed to send letters',
+    ),
+    (
+        {'restricted': True},
+        403,
+        'Cannot send letters when service is in trial mode',
+    )
 ])
 def test_post_letter_notification_returns_403_if_not_allowed_to_send_notification(
     client,
     notify_db_session,
-    service_args
+    service_args,
+    expected_status,
+    expected_message,
 ):
     service = create_service(**service_args)
     template = create_template(service, template_type=LETTER_TYPE)
@@ -480,10 +490,10 @@ def test_post_letter_notification_returns_403_if_not_allowed_to_send_notificatio
         'personalisation': test_address
     }
 
-    error_json = letter_request(client, data, service_id=service.id, _expected_status=400)
-    assert error_json['status_code'] == 400
+    error_json = letter_request(client, data, service_id=service.id, _expected_status=expected_status)
+    assert error_json['status_code'] == expected_status
     assert error_json['errors'] == [
-        {'error': 'BadRequestError', 'message': 'Service is not allowed to send letters'}
+        {'error': 'BadRequestError', 'message': expected_message}
     ]
 
 

tests/app/v2/notifications/test_post_notifications.py

@@ -223,7 +223,7 @@ def test_should_cache_template_lookups_in_memory(mocker, client, sample_template
         'template_id': str(sample_template.id),
     }
 
-    for i in range(5):
+    for _ in range(5):
         auth_header = create_authorization_header(service_id=sample_template.service_id)
         client.post(
             path='/v2/notifications/sms',