Support context manager protocol by contextvars.Token

Feature or enhancement

Sometimes, mostly in tests, I write something like

from contextvars import ContextVar
var = ContextVar('var')

def test_a():
    token = var.set('new val')
    do_stuff()
    var.reset(token)

It looks a little cumbersome, the support for with var: would be awesome:

def test_b():
    with var.set('new val'):
        do_stuff()

This is a minor feature, which does not need previous discussion elsewhere

No response

#129888 provides a simple patch for the proposed functionality.

gh-129889: Support context manager protocol by contextvars.Token (#12…

Related but bigger: #99633