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/authentication/test_authentication.py

@@ -479,7 +479,7 @@ def test_should_cache_service_and_api_key_lookups(mocker, client, sample_api_key
         wraps=dao_fetch_service_by_id,
     )
 
-    for i in range(5):
+    for _ in range(5):
         token = __create_token(sample_api_key.service_id)
         client.get('/notifications', headers={
             'Authorization': f'Bearer {token}'