Use abc module to signal abstract base classes

The Completer class is abastract, and it's complete method is also
abstract. Using ABCMeta achieve two thing, first it will cause an error
if a developer doesn't overwrite the method in a subclass. And second
that it tells the static analysis tools to ignore the unused arguments
in the signature, since this is the definition of a signature, not an
actual function.

By the same logic addressbook.AddressBook, and account.Account have been
extended to use ABCMeta as their metaclass and have had their abstract
methods decorated with the abstract method attribute.

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