Add asgiref to EXTERNAL_REQ_ALLOWLIST

Hi, is it ok to add `asgiref` package to the EXTERNAL_REQ_ALLOWLIST. As I mentioned in the PR [typeshed-internal/stub_uploader#173](https://github.com/typeshed-internal/stub_uploader/pull/173), the package is in the [PyPI top 1000 list](https://hugovk.github.io/top-pypi-packages/) (ranked 195th). It is fully type-hinted and under the Django umbrella, so I think it’s safe to add to the EXTERNAL_REQ_ALLOWLIST.