Check if user nickname is an available assignee for the repo before setting assignee

[awdavis]

Currently, the script checks for whether 'https://api.github.com/users/' + username gets a 200 response. If so, it will set the username (or nickname) as the GitHub issue assignee.

The problem is the user profile might return 200, but still not be assignable in the repo, which will cause the script to fail with an error like the following:

RuntimeError: Failed to import GitHub issue due to the following errors:
{
  'id': 9969067,
  'status': 'failed',
  'url': 'https://api.github.com/repos/OWNER/REPO/import/issues/9969067',
  'import_issues_url': 'https://api.github.com/repos/OWNER/REPO/import/issues',
  'repository_url': 'https://api.github.com/repos/OWNER/REPO',
  'created_at': '2024-06-19T18:23:13.000-07:00',
  'updated_at': '2024-06-19T18:23:13.000-07:00',
  'errors': [{'location': '/issue/assignee', 'resource': 'Issue', 'field': 'assignee', 'value': 'USER_NICKNAME_HERE', 'code': 'invalid'}]
}

The solution may be to use the GitHub API to list the available assignees, or check if a user is assignable, in that specific repo:

• https://docs.github.com/en/rest/issues/assignees?apiVersion=2022-11-28#check-if-a-user-can-be-assigned
• https://docs.github.com/en/rest/issues/assignees?apiVersion=2022-11-28#check-if-a-user-can-be-assigned