[FIX] website_customer: make res.partner.tag.classname overridable

The selection function get_selection_class for the field classname has
obviously been thought to be overridable by subclasses in order to add
new selection classes if need be.

However, before this commit the method was passed directly to the
Selection constructor, which used the callable object directly. If it
was overridden by a subclass, the Selection object would still use the
same non-overridden callable instance.

With this commit, we give the name of the method instead of the
callable, which makes that the method is resolved after all overrides,
and therefore the resulting selection will be the overridden one.

closes odoo/odoo#122299

Signed-off-by: Romain Derie (rde) <rde@odoo.com>