Investigate switching to urllib3

[jku]

We currently use requests in the default ngclient fetcher (RequestsFetcher).

Internally requests uses urllib3 and we don't seem to use any functionality outside of what urllib3 provides: connection pool management and streaming are supported. Switching to urllib3 would mean:

• we would have (at least) one less dependency
• the API we would use is arguably better maintained

This could be achieved by first writing an alternative FetcherInterface implementation that uses urllib3: if it looks good we can then make it the default and remove the requests-based implementation.