Team class using legacy API endpoints

[mxmehl]

I noticed that the Team class uses some legacy endpoints.

One example is Team.add_membership(), which calls PUT /teams/{id}/memberships/{user}. According to the REST API documentation, this endpoint is legacy:

Deprecation Notice: This endpoint route is deprecated and will be removed from the Teams API. We recommend migrating your existing code to use the new Add or update team membership for a user endpoint.

PUT /orgs/{org}/teams/{team_slug}/memberships/{username}

Is this already on your radar? How do you generally handle such deprecations?

If this is a todo item for you, I'm not sure whether I can execute it alone. I already made a smaller contribution to this project, but I wouldn't know how to make such a big transition as definitely more than just this endpoint is deprecated.

[EnricoMi]

I think that should simply be fixed by changing the URL used. Updating the test data might be the most elaborate part.

You could draft a PR changing the URLs first and I can look into test data. Please consider all /team/{id} urls.