given the user login, looks for a user in assignee list of the repo
        and return it if was found.

def get_assignee(self, login):
        """
        given the user login, looks for a user in assignee list of the repo
        and return it if was found.
        """
        if not login:
            return GithubObject.NotSet
        if not hasattr(self, '_assignees'):
            self._assignees = {c.login: c for c in self.repo.get_assignees()}
        if login not in self._assignees:
            # warning
            print("{} doesn't belong to this repo. This issue won't be assigned.".format(login))
        return self._assignees.get(login)