def user_principals(self, username):
        principals = ['user:' + username, Everyone, Authenticated]
        if self.is_admin(username):
            principals.append('admin')
        for group in self.groups(username):
            principals.append('group:' + group)
        return principals
Provide a list of all oembed providers that are being used.
dataset/ETHPy150Open mathcamp/pypicloud/pypicloud/access/base.py/IAccessBackend.user_principals--crossover with--dataset/ETHPy150Open worldcompany/djangoembed/oembed/sites.py/ProviderSite.get_providers