7.0 refactor account_partner_required

refactor account_partner_required:
* add extension points for policies
* use constraints for readbility and robustness
* slightly adapt the test to work around an issue in account_constraints (which allows the simplification of the travis configuration)
This is setting the `id(account_account_type.__get_policies)` as the function, which means you cannot override it.

Using a lambda `self, *a, **kw: self.__get_policies(*a, **kw)` sets the function as the `id` of the lambda which calls on `self` which can be overridden.

In 8.0, you don't need this when you pass the function name as a string.