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/dao/notification_dao/test_notification_dao.py

@@ -511,7 +511,7 @@ def test_get_notification_for_job(sample_notification):
 
 
 def test_get_all_notifications_for_job(sample_job):
-    for i in range(0, 5):
+    for _ in range(0, 5):
         try:
             create_notification(template=sample_job.template, job=sample_job)
         except IntegrityError:
@@ -546,7 +546,7 @@ def test_dao_get_notification_count_for_job_id(notify_db_session):
     service = create_service()
     template = create_template(service)
     job = create_job(template, notification_count=3)
-    for i in range(3):
+    for _ in range(3):
         create_notification(job=job)
 
     create_notification(template)