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

app/dao/users_dao.py

@@ -23,12 +23,12 @@ def create_secret_code():
     return ''.join(map(str, [SystemRandom().randrange(10) for i in range(5)]))
 
 
-def save_user_attribute(usr, update_dict={}):
-    db.session.query(User).filter_by(id=usr.id).update(update_dict)
+def save_user_attribute(usr, update_dict=None):
+    db.session.query(User).filter_by(id=usr.id).update(update_dict or {})
     db.session.commit()
 
 
-def save_model_user(user, update_dict={}, password=None, validated_email_access=False):
+def save_model_user(user, update_dict=None, password=None, validated_email_access=False):
     if password:
         user.password = password
         user.password_changed_at = datetime.utcnow()
@@ -36,7 +36,7 @@ def save_model_user(user, update_dict={}, password=None, validated_email_access=
         user.email_access_validated_at = datetime.utcnow()
     if update_dict:
         _remove_values_for_keys_if_present(update_dict, ['id', 'password_changed_at'])
-        db.session.query(User).filter_by(id=user.id).update(update_dict)
+        db.session.query(User).filter_by(id=user.id).update(update_dict or {})
     else:
         db.session.add(user)
     db.session.commit()