Leaking file descriptors

There are two places where file descriptors are being leaked , at least with python 3.8:

1. _get_raw_response_socket(): in case of Unix sockets, additional referencing seems to stop them from being closed
2. When DockerClient is not used as a context manager it does not call close() on the api session object.

I have managed to work around with:

class FixedAPIClient(docker.APIClient):
    def _get_raw_response_socket(self, response):
        sock = super()._get_raw_response_socket(response)
        if self.base_url.startswith('http+docker://'):
            sock._response = None
        return sock


class FixedDockerClient(docker.DockerClient):
    def __init__(self, *args, **kwargs):
        self.api = FixedAPIClient(*args, **kwargs)

    def __del__(self):
        try:
            if self.api:
                self.api.close()
        except AttributeError:
            pass

Our setup is quite complex. If you believe this should not happen and it is a corner case I will try to come up with a minimal example.

Start by reducing the reported setup to a minimal reproduction for the two paths: _get_raw_response_socket() with Unix sockets and DockerClient used without a context manager. Inspect how the API session is created and closed, then verify that both paths release their file descriptors without relying on the workaround shown.