trio.Cancelled can't be pickled

[hotpxl]

trio.Cancelled can't be constructed directly so it can't be pickled. Usually this is fine as no one needs to pickle and move around a trio.Cancelled object. But sometimes it gets trapped in some other exception's __context__ or __cause__.

One example is

async def test():
    with trio.fail_after(0.1):
        await trio.sleep(5)

try:
    trio.run(test)
except trio.TooSlowError as e:
    print(e.__context__)

(By default __context__ of an exception doesn't get pickled but tblib.pickling_support adds it, which I think is reasonable?)

And now we can't pickle this TooSlowError anymore.

Here's the full example:

from tblib import pickling_support
pickling_support.install(trio.TooSlowError)

async def test():
    with trio.fail_after(0.1):
        await trio.sleep(5)

try:
    trio.run(test)
except trio.TooSlowError as e:
    b = e

import pickle
pickle.loads(pickle.dumps(b))

Can we add pickle support for trio.Cancelled?

[A5rocks]

Prior discussion in #3105. (well "discussion" as in @jakkdl saying this would be nice polish :^)