Type check glcollate/glproxy.py

Make __build_gitlab_object a chain of static functions that return Gitlab object because it has to be called at initialization. Most type checkers only consider attributes assigned in __init__ and will raise errors if any attributes are first assigned outside it.

The Gitlab.user attribute can be None so check that it is not None in __init__ and assign a non-None value to the _user attribute. Raise ValueError if user was not retrieved.

Depends on !70 (merged)