pair_with_previous method for Observable instances in RxPy

pairwithprevious.py

from rx.core import Observable, AnonymousObservable
from rx.internal import extensionmethod

@extensionmethod(Observable)
def pair_with_previous(self):
  """
  stream = Observable.from_iterable(
    list(map(
      lambda x: [1*x, 2*x],
      range(6)))
  )

  stream = stream.pair_with_previous()

  stream.subscribe(print)
  """

  return self.scan(
    lambda acc, current: (acc[1], current),
    (None, None)
  )