Support for async-safety through asgiref's Local

We are looking to switch to ASGI soon and are freshly using django-multitenant as well. As the `threading.local` is being used by django-multitenant, I'm afraid of cross-request tenant leakages.
I therefore suggest an [approach similar to django-simple-history](https://github.com/jazzband/django-simple-history/blob/e368d839c981c60376cbffcdaba245b89766bff0/simple_history/models.py#LL43C1-L46C48):

```
try:
from asgiref.local import Local as LocalContext
except ImportError:
from threading import local as LocalContext
```

This should use asgiref's `Local` as an asgi-safe alternative to `threading.locals` as per my understanding. As asgiref.local is meant as a drop-in replacement, the change should be straightforward.