* Fixes #929 : error in Client.activate_session()

Fixes the error that a call to Client.activate_session() without arguments overwrites a user certificate previously set with Client.load_client_certificate(user_cert_file).

* Do not override already set user certificate

If you set the user certificate with Client.load_client_certificate() and then activate the session with Client.activate_session() without passing the certificate as an argument, the previously set certificate is discarded.

#922

see. https://github.com/FreeOpcUa/opcua-asyncio/pull/878#issuecomment-1120423672

- help for type check
- highlight function arguments types

Closes #885

use unittest AsyncMock instead of CoroutineMock which creates warnings

DeprecationWarning: The explicit passing of coroutine objects to asyncio.wait() is deprecated since Python 3.8, and scheduled for removal in Python 3.11.

and tests