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/aws/test_s3.py

@@ -13,11 +13,11 @@ from app.aws.s3 import (
 from tests.app.conftest import datetime_in_past
 
 
-def single_s3_object_stub(key='foo', last_modified=datetime.utcnow()):
+def single_s3_object_stub(key='foo', last_modified=None):
     return {
         'ETag': '"d41d8cd98f00b204e9800998ecf8427e"',
         'Key': key,
-        'LastModified': last_modified
+        'LastModified': last_modified or datetime.utcnow(),
     }