As discussed over the mailing list, reformat code using Black. This is a
one-off change to reformat all our codebase. Moving forward, we
shouldn't expect such blanket reformats. Rather, we expect each change
to already comply with the Black formatting style.

It seems we don't need this anymore, thanks to cleaning up gRPC
background threads.

tests: manually close channels when interacting with the cache

cascache.py: disable the usage monitor if start method is spawn

Additionally, test code that was previous executed by a subfunction (in
the forked process) has been folded into the test function itself, as
separating it is now redundant. This removes some duplicate code for
setting up the context and project, etc.

Now that both are equivalent, we can skip the sanitization part before
the yaml call.

Instead of having methods in Context forward calls on to the Messenger,
have folks call the Messenger directly. Remove the forwarding methods in
Context.

Remove the need to pass the Context object to message handlers, by
passing what is usually requested from the context instead.

This paves the way to sharing less information with some child jobs -
they won't need the whole context object, just the messenger.

Calling '_yaml.dump' will itself call '_yaml.node_sanitize', therefore
we can remove all calls to it in places where we directly after call
dump.