>>> dt = pendulum.parse("2017-09-01 00:00:00+00:00")
>>> dt
DateTime(2017, 9, 1, 0, 0, 0, tzinfo=Timezone('+00:00'))
>>> dt.format('%Y-%m-%dT%H:%M:%S.%fZ', formatter='classic')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: format() got an unexpected keyword argument 'formatter'

Per #255 it is mentioned the keyword is not supported anymore, it then should be removed from the documentation.