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_users_dao.py

@@ -124,12 +124,12 @@ def test_should_not_delete_verification_codes_less_than_one_day_old(sample_user)
     assert VerifyCode.query.one()._code == "12345"
 
 
-def make_verify_code(user, age=timedelta(hours=0), expiry_age=timedelta(0), code="12335", code_used=False):
+def make_verify_code(user, age=None, expiry_age=None, code="12335", code_used=False):
     verify_code = VerifyCode(
         code_type='sms',
         _code=code,
-        created_at=datetime.utcnow() - age,
-        expiry_datetime=datetime.utcnow() - expiry_age,
+        created_at=datetime.utcnow() - (age or timedelta(hours=0)),
+        expiry_datetime=datetime.utcnow() - (expiry_age or timedelta(0)),
         user=user,
         code_used=code_used
     )