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_reporting_tasks.py

@@ -212,7 +212,7 @@ def test_create_nightly_billing_for_day_different_sent_by(
     records = FactBilling.query.order_by('rate_multiplier').all()
 
     assert len(records) == 2
-    for i, record in enumerate(records):
+    for _, record in enumerate(records):
         assert record.bst_date == datetime.date(yesterday)
         assert record.rate == Decimal(1.33)
         assert record.billable_units == 1
@@ -226,7 +226,7 @@ def test_create_nightly_billing_for_day_different_letter_postage(
     yesterday = datetime.now() - timedelta(days=1)
     mocker.patch('app.dao.fact_billing_dao.get_rate', side_effect=mocker_get_rate)
 
-    for i in range(2):
+    for _ in range(2):
         create_notification(
             created_at=yesterday,
             template=sample_letter_template,