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/test_invited_user_dao.py

@@ -138,13 +138,13 @@ def test_should_not_delete_invitations_less_than_two_days_old(
     assert InvitedUser.query.first().email_address == "valid@2.com"
 
 
-def make_invitation(user, service, age=timedelta(hours=0), email_address="test@test.com"):
+def make_invitation(user, service, age=None, email_address="test@test.com"):
     verify_code = InvitedUser(
         email_address=email_address,
         from_user=user,
         service=service,
         status='pending',
-        created_at=datetime.utcnow() - age,
+        created_at=datetime.utcnow() - (age or timedelta(hours=0)),
         permissions='manage_settings',
         folder_permissions=[str(uuid.uuid4())]
     )