tls: use auto-negotiated highest version (#3007)
tls: use auto-negotiated highest version (#3007)

Specific TLS versions are deprecated in latest Python, which
causes test failures due to treating deprecation errors as
warnings.

Luckily, the fix here is straightforward: we can eliminate some
custom version selection logic by using `PROTOCOL_TLS_CLIENT`,
which is the recommended method and will select the highest TLS
version supported by both client and server.

Signed-off-by: Milas Bowman <bf766bbdae72109c448c7f902860a03abdcfef7b@docker.com>
docker/docker-py,docker/docker-py