Return correct object as client protocol

[asikeero]

Fixes #820

[PierreF]

Thanks for catching this error. Could you sign ECA (https://accounts.eclipse.org/user/eca / https://www.eclipse.org/legal/ECA.php) so I can merge you PR ?

[asikeero]

Done, should be OK now.

[asikeero]

Two checks are failing due to reasons that seem not to relate to this PR. Just to clarify, am I expected to add something to this PR to make them pass?

[PierreF]

Thanks for your patience. Pytest (tox) tend to be have false-positive errors (cf #789). The lint_python error is a real error, linked to your change.
Your change isn't the issue actually, but make the issue visible: self._protocol is not correctly typed.

I'll merge your PR and fix the typing once merged.

Thanks for your contribution.