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/notifications/test_validators.py

@@ -118,7 +118,7 @@ def test_should_set_cache_value_as_value_from_database_if_cache_not_set(
 ):
     serialised_service = SerialisedService.from_id(sample_service.id)
     with freeze_time("2016-01-01 12:00:00.000000"):
-        for x in range(5):
+        for _ in range(5):
             create_notification(sample_template)
         mocker.patch('app.notifications.validators.redis_store.get', return_value=None)
         mocker.patch('app.notifications.validators.redis_store.set')
@@ -149,7 +149,7 @@ def test_check_service_message_limit_over_message_limit_fails(key_type, sample_s
         template = create_template(sample_service)
         serialised_service = SerialisedService.from_id(sample_service.id)
 
-        for x in range(5):
+        for _ in range(5):
             create_notification(template)
         with pytest.raises(TooManyRequestsError) as e:
             check_service_over_daily_message_limit(key_type, serialised_service)