Make dispatcher use one context per update

[Eldinnie]

It gives user the option to overload context with their own properties in a lower group handler if they like

[jsmnbom]

This looks good to me. A bit cluttered with the flake8 stuff (would've been nice to merge that to master first, so our builds there don't fail all the time, but mweh)

1. We should document this behaviour somewhere.
2. Should context maybe be reset after each handler? Like resetting only the standard attributes. It just feels a bit strange to me that (using test_one_context_per_update as example) I would be able to access context.matches in two, even though it does not have a regex filter (and therefore no matches).
3. This is all still threadsafe right?

[Eldinnie]

1. Yes in the wiki I guess?
2. I think that's overthingking it personally...
3. ehm.... I guess it might not be on run_async handlers....

[jsmnbom]

1. maybe in the ContextCallback class too? Like "You can add attributes to this class and they will propagate to handlers in higher dispatcher groups."
2. Alright, I do think we should add another note to somehow describe this behaviour
3. We need to test that somehow I think

[jsmnbom]

Okay, so this feature simply doesn't work with @run_async at all... Is that okay?

Test code to show not working/working (uncomment/comment the @run_async

# @run_async
def a(u, c: CallbackContext):
    print(c)
    c.q = 1

# @run_async
def b(u, c: CallbackContext):
    print(c)

u = Updater(token, use_context=True)
d = u.dispatcher
d.add_handler(MessageHandler(Filters.all, b))
d.add_handler(MessageHandler(Filters.all, a), -1)

u.update_queue.put(Update(0, message=Message(0, None, None, None)))

u.start_polling()
u.idle()