rbintegrations was updated to switch to pyproject.toml, which uses

the package backend in Review Board. However, this means it needs to be

able to pull down Review Board in an isolated build environment in order

to get to the package backend. This isn't feasible today, since that's

not released, and it always means we'll be using the released and not

in-development version of Review Board.
Ideally we'd be able to use .local-packages to point to Review Board

here, but that's not viable because we can't use buildthings for this.

In the future, there will be an rbext package built on that, but that

does us no good right now.
However, we can restore setup.py and use that directly to kick off the

build. This effectively removes the isolated build environment, so long

as we run setup.py instead of python -m build. This isn't going to

work forever, but it's the same approach we use for all other

extensions.
This setup.py is minimal. In modern setuptools, it will kick off the

build but pull all metadata from pyproject.toml. This gives us a

bridge toward modern builds but restores the ability to deploy and

release this.

Successfully built packages manually and with beanbag-tools.
Note that python -m build will still fail until Review Board 8

is released.