Eliminate requests dependency

[uogbuji]

The function from librarylink.util.all_sites still uses the requests library. Switch to aiohttp. We're already

Note: This will mean updating minimal Python support to 3.5 (limitation comes from latest aiohttp version)

Note: Right now there is a bug where setup.py is missing the requests dependency. That will be addressed by this as well.

[uogbuji]

Actually I'm going to push Python minimum to 3.6, in order to use the asynchronous generator pattern. I'd not wanted to push beyond the Debian stable threshold (3.5), but the pragmatics of not having to put off that refactoring wins for me.