Using contextvars directly instead of aiocontextvars

[RouquinBlanc]

We use contextvars internally for tracking the current active Scope.

This feature was introduces in python 3.7 with full asyncio support, and although an official backport exist, it is still not fully supporting asyncio (MagicStack/contextvars#2).

In the mean time, we rely on aiocontextvars which does pretty much the job, but with yet no background on how and when full asyncio support will be released. This project itself is marked as pending a fix on the official backport.