Add support for using Enum to declare states.

[fgmacedo]

Description

Add support for using Enum to declare states.

Something like this:

from enum import Enum
from enum import auto

from statemachine import State
from statemachine import States
from statemachine import StateMachine


class TrafficLight(Enum):
    green = auto()
    yellow = auto()
    red = auto()


class TrafficLightMachine(StateMachine):
    "A traffic light machine"

    states = States.from_enum(TrafficLight, initial=TrafficLight.green)

    cycle = (
        states.green.to(states.yellow)
        | states.yellow.to(states.red)
        | states.red.to(states.green)
    )

    def before_cycle(self, event: str, source: State, target: State):
        return f"Running {event} from {source.id} to {target.id}"

What do you think?

[fgmacedo]

Implemented: #375