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.

This reverts commit b54c8cb07ca257be79940ffa70853bf75d2c287c.

Part of #1044

If the cached Artifact wasn't originally generated with a buildtree,
then there's no need to attempt to find it in remotes by entering
Stream(). In interactive mode we only present the user the option to
attempt the pull with the above assumption on buildtree_exists, so
apply the same to applicable non interactive choices.

This also includes some tweaks in integration/shellbuildtrees.py
to reflect changes and cover cases that were missing.

For some reason PROG_NOT_FOUND and APP were both under 12 which could be
confusing. Changed PROG_NOT_FOUND to 16 and added a unique decorator to
prevent this from happening again.

Updated shellbuildtrees test to check correct error domain.

We have two different 'site' files that are redundant and both
define some variables in BuildStream environment.

Remove HAVE_SANDBOX from tests/testutils/site.py and change all imports
to point to the other one