.arcconfig

peer: make ui an attribute

With abc interfaces, instance attributes could not satisfy
@abc.abstractproperty requirements because interface conformance
was tested at type creation time. When we created the abc
peer interfaces, we had to make "ui" a @property to satisfy
abc.

Now that peer interfaces are using zope.interface and there is no
import time validation (but there are tests validating instances
conform to the interface), we can go back to using regular object
attributes.

Differential Revision: https://phab.mercurial-scm.org/D3069

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