zope.{interface,component} implement Interfaces through __implements__,
__implemented__ (both implementedBy instances) and __provides__ (ClassProvides
instance) attributes set on the class itself through implementedByFallback
(zope.interface.declarations).

However, this implementation creates circular references by referencing the
class itself and thus erp5.* classes (and all its Accessors instances) were
never GC even after a reset.

When running testXHTML and installing the Unit Tests bt5s:
  * After 10 resets:  21MB leak (~  7% of Zope process USS).
  * After 20 resets:  70MB leak (~ 18% of Zope process USS).
  * After 28 resets: 122MB leak (~ 26% of Zope process USS).