DockerEvents impossible to restart

[zentarim]

Long story short

DockerEvents objects can be opened but it cannot be reopened again.
Pull request will be created based on this issue.

• Expected behaviour:

await subscribe()    # the self.task is started
await stop()         # the self.task is cancelled and wiped
await subscribe()    # the self.task is started

• Actual behaviour:

await subscribe()    # the self.task is started
await stop()         # the self.task is cancelled but is not wiped
await subscribe()    # self.task is not started. No log data will be collected.

How to reproduce

Can be reproduced at the test_events test start as:

    # Сheck the stop procedure
    docker.events.subscribe()
    await docker.events.stop()

    subscriber = docker.events.subscribe()
    # test body

Your environment

A Centos\Ubuntu Linux various versions.
Python 3.6, 3.7, 3.8