Fix Root.add_key() argument's type

[sechkova]

Fixes #1378

Description of the changes being introduced by the pull request:

Since the existence of a Key class, taking Key as an argument type seems like the logical change.
The test case is updated accordingly.

Since there are no "users" of this functionality yet, I can't say if further improvements are
needed for now.

Please verify and check that the pull request fulfills the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[sechkova]

Force-pushed after I changed my mind over the test case a bit

[jku]

looks ok to me but could you add a regression test so root metadata with an added key serializes correctly (a test that fails with current develop and succeeds with your fix)?

[sechkova]

looks ok to me but could you add a regression test so root metadata with an added key serializes correctly (a test that fails with current develop and succeeds with your fix)?

I was wondering whether to add such tests case because the bug was in our test using incorrectly the class ... and in the type annotation but types in Python ...
Anyway, since this comes up as a request, I guess adding it was the better decision. I'll update the PR.

[jku]

I was wondering whether to add such tests case because the bug was in our test using incorrectly the class

yeah you do have a point here -- thanks anyway.