The Django API requires that registered checks take an "app_configs"
argument, which is a list of AppConfig. This is practically also passed by
keyword, so all parameters should be specified with a "= ..." to indicate
that they are keywords.
Django always passed "app_configs" as a list, but checks can accept and
use a more general more Sequence.
The Django API also requires that registered checks take "**kwargs".
This results in the canonical parameters of:
"app_configs: Optional[Sequence[AppConfig]] = ..., **kwargs: Any"
* Allow overridable checks to be CheckMessage
The checks framework expects return types to be "List[CheckMessage]". This
allows all overridable checks-returning methods to return the more general
"CheckMessage", instead of the "Error" subclass.
* Ignore an incorrect usage of the checks API in Django 2.2