The PyPI Legacy API documentation [1] says that the distribution
downloads for project can be accessed via a request
GET /simple/<project>/
but the current implementation instead makes a request
GET /simple/<project>
There exist some custom repository servers that give erroneous responses
to requests without the slash, so include the slash.
[1]: https://warehouse.pypa.io/api-reference/legacy.html
We want this fetcher to still work even when the CA certs and server
certs are expired or otherwise considered insecure for backwards
compatibility reasons.