Use contextvars instead of thread.local to support asyncio task-local logging

[n1ywb]

Python 3.7 added a new contextvars module to present a unified context-local storage API for both threaded and async programs.

https://docs.python.org/3.7/library/contextvars.html

Context managers that have state should use Context Variables instead of threading.local() to prevent their state from bleeding to other code unexpectedly, when used in concurrent code.

This seems to tie into the ongoing async logging work

[hynek]

structlog had support for contextvars since 20.1: http://www.structlog.org/en/stable/contextvars.html

[n1ywb]

thanks, I had missed that. closing.