Tekore version 2.0

[felix-hilden]

This pull request will introduce the next iteration of Tekore. As such, it contains a fair amount of breaking changes.

• What should be importable from the top level #81 Remove submodules
• Use HTTPX exclusively, sender changes #139 Drop requests in favor of HTTPX
• Refactor sender module #140 Refactor sender module
• Use persistent senders by default #141 Default to persistent senders
• Model consistency, serialisation and methods #149 Model methods
• Relocate errors #154 Relocate errors
• Playlist track, episode and is_local #170 Playlist item attributes
• Parse token scopes and improve scope operations #177 Parse scopes and improve scope operations
• Playlist now fully support episodes #178 Full episode support for playlists
• Close Sessions in PersistentSender #179 Close sessions in PersistentSender
• Announce Tekore 2.0 at least on RTD home page

We'll also finally switch test runners from the builtin unittest to pytest mainly for the ease of declaring asynchronous tests with pytest-asyncio. Mocking and patching is still done with unittest though, and we ought to consider changing that. But this is a start.

[felix-hilden]

@marioortizmanero I would really appreciate if you had a quick look at these changes, just to get a second opinion on our general direction. A full code review is a bit much to ask, but looking at the linked issues, release notes, and even the documentation provides a good overview.

Also, I think we'll go with Requests+HTTPX for now. It seems to be working alright.

[felix-hilden]

Would you be willing to do it, @marioortizmanero? If not, say, during this week, then I'll merge these changes and make a release soon.