Code review comment for lp:~canonical-platform-qa/autopilot/cpo-base-classes-fix

Follow up:
I ran the fixed (i.e. Leos base_emulator update branch) and non fixed address book tests, the fixed worked well, the non-fixed exhibited the error we see.

We should remove the additional code that removes the method attribute and change the warning into an error (probably RuntimeError? or ValueError?).