Make `DockerClient` a Context Manager

It's not obvious that DockerClient needs to have its close() method called to clean up resources, which can lead to unclosed sockets. We noticed this using asyncio and pytest together, which complains about unclosed sockets when tearing down test sessions.

Since DockerClient is wrapping ApiClient, and ApiClient inherits its close() method from requests.Session, it's not even obvious from a brief glance source what close() is for.

I expect that ApiClient is already a Context Manager, since it would inherit __enter__ and __exit__ from requests.Session, so the low-level API already has this behaviour.

To make it easier to avoid mistakes, DockerClient could have __enter__ and __exit__ methods added, identical to requests.Sessions, which would allow safer usage patterns such as

with docker.from_env() as docker_client:
 # Do stuff with docker_client

when that's feasible.