Error when using spack create with backports.ssl_match_hostname

When using `spack create` with backports.ssl_match_hostname, the following error occurs:

```
spack -d -v create https://pypi.io/packages/source/b/backports.ssl_match_hostname/backports.ssl_match_hostname-3.5.0.1.tar.gz
==> Error: Unknown namespace: backports
```