v4.0 - requests, niquests, httpx or something else?

[tobixen]

This is a continuation of #457

My final conclusion in #457 was that I would make niquests an explicit dependency in pyproject.toml, but since this may be controversial (ref #530) ensure that things still works if someone forks the project and simply. I also conluded that I would reconsider before releasing v4.0.

(v3.0 is still on the drawing board, v4.0 is maybe half a year or a year ahead, possibly more at the time of writing - but I still leave it here as a topic that needs to be discussed/decided).