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/celery/test_tasks.py

@@ -1475,7 +1475,7 @@ def test_send_inbound_sms_to_service_does_not_sent_request_when_inbound_api_does
     mocked = mocker.patch("requests.request")
     send_inbound_sms_to_service(inbound_sms.id, inbound_sms.service_id)
 
-    mocked.call_count == 0
+    assert mocked.call_count == 0
 
 
 def test_send_inbound_sms_to_service_retries_if_request_returns_500(notify_api, sample_service, mocker):
@@ -1525,7 +1525,7 @@ def test_send_inbound_sms_to_service_does_not_retries_if_request_returns_404(not
                           status_code=404)
         send_inbound_sms_to_service(inbound_sms.id, inbound_sms.service_id)
 
-    mocked.call_count == 0
+    assert mocked.call_count == 0
 
 
 def test_process_incomplete_job_sms(mocker, sample_template):