This introduces a new alternative to spy_on(), the @spy_for()

decorator. This is is a convenience around creating a function and then

spying using call_fake=, simplifying test code by a few lines.
@spy_for() takes a function/method to spy on and an optional owner,

and will take care of setting up a spy that passes the decorated

function as call_fake=.
For example:

def test_doomsday_device(self):
    dd = DoomsdayDevice()

    @self.spy_for(dd.kaboom)
    def _save_world(*args, **kwargs)
        print('Sprinkles and ponies!')


Which is simpler and more compact than:

def test_doomsday_device(self):
    dd = DoomsdayDevice()

    def _save_world(*args, **kwargs)
        print('Sprinkles and ponies!')

    self.spy_on(dd.kaboom, call_fake=_save_world)


The README has been updated to document this new usage, and to prefer it

in examples.

Unit tests pass on all supported versions of Python.