Behaviour of ObservableMixin when its `off` method is called while its `fire` method is currently iterating over listeners

Suppose we have two listeners on "close" event.

import txaio
from autobahn.util import ObservableMixin

txaio.use_twisted()

class Foo(ObservableMixin):

    def __init__(self):
        self.set_valid_events(["someEvent"])

    def doSomething(self):
        self.fire("someEvent", self)


def listener1(foo):
    f.off("someEvent", listener1)
    print("listener1 called")

def listener2(foo):
    print("listener2 called")

f = Foo()

f.on("someEvent", listener1)
f.on("someEvent", listener2)

f.doSomething()

This will output:

listener1 called

This means, when ObservableMixin off method is called inside listener1,
except for last listeners, then the next listeners (listener2) will never get called.

I think this is because ObservableMixin off method is called while
ObservableMixin fire method is iterating over current listeners of event..

Locate ObservableMixin's fire and off implementations and reproduce the issue with the two-listener example from the report. Add a regression test covering a listener removing itself during dispatch; done means the remaining listener is still called.