No download URL for PyPI

Applying get_download_url() to
```
pkg:pypi/django@1.11.1
```
gives no result.

I would have expected something like:
```
https://files.pythonhosted.org/packages/22/68/24fd855343f218e016bbb04b1a5a2dc1be191a4cbd4d3cdabb13d1c2a371/Django-1.11.1.tar.gz
```
