Gogs issue creation fails when assignee specified

[kbenzie]

$ git issue create -a benie
Traceback (most recent call last):
  File "/home/benie/.local/bin/git-issue", line 8, in <module>
    sys.exit(main())
  File "/home/benie/.local/lib/python3.8/site-packages/git_issue/cli.py", line 508, in main
    command(get_service(), **args)
  File "/home/benie/.local/lib/python3.8/site-packages/git_issue/cli.py", line 273, in create
    issue = service.create(
  File "/home/benie/.local/lib/python3.8/site-packages/git_issue/gogs.py", line 64, in create
    assignee = _check_assignee_(kwargs.pop('assignee', None))
  File "/home/benie/.local/lib/python3.8/site-packages/git_issue/gogs.py", line 21, in _check_assignee_
    raise ValueError('assignee must be an instance of GogsUser')
ValueError: assignee must be an instance of GogsUser