py3: don't use dict.iterkeys()
Instead of iterating of dict.iterkeys(), let's iterate of dict as that will be
equivalent and dict.iterkeys() is not present in Python 3.
Differential Revision: https://phab.mercurial-scm.org/D1796
public attributes not in abstract interface: badpeer.badattribute
public attributes not in abstract interface: badpeer.badmethod