[Backport] Allow minus character in ProductAttribute.code

I don't see a reason why it shouldn't be allowed. And as the Django
Admin is currently the only way to enter product attributes, supporting
it eases integration with the auto-slugifying that is enabled in
ProductAttributeAdmin.

Fixes #1308

(cherry picked from commit 4491582979a9515f555c56d714c67f2fdac81ea7)