Maybe more test with a re-entrant context? as well as a test when an exception is raised in the context body?

Thanks for the review, I'll add tests.
I don't expect any problems though since the implementation is really trivial.

Up to you though! Generally, I'm not actually adding those kind of tests to test the implementation but rather to spot possible regressions if we change something. I actually don't know if we are that pedantic when testing other context managers so feel free not to burden the tests if you think it's not worth it!

Tests are cheap and easy to write,
I've added two: one for exit when an exception is raised and another for reentrancy.

Please feel free to ask for additional tests if needed.

Looks good! Small question, but is there a necessity to check what happens if we callc.reset() inside the context manager? or multiplec.set() as well?

Multiplec.set() doesn't affect the context manager, the value will be reset on exit.
c.reset() is safe if the other token was used; resetting with the same token raises RuntimeErorr as in the following example:

with c.set(1) as token:    c.reset(token)

A couple additional tests were added to demonstrate mentioned scenarios.