With legacy packaging centered around setup.py now on the way out, we

need to move to modern packaging using pyproject.toml and build

backends. The challenge is that Review Board's build setup is a bit more

involved than your average Python package, with static media building

and gettext i18n file generation being part of the process, and a fair

amount of setup required to get there.
This change introduces a build backend based on the work we did for

Djblets's backend, but with a few extra capabilities to ease

development and support the setup requirements, and doing so in pip's

isolated build environment.
The build backend handles a few custom steps:

Dependency management
Static media setup and building
i18n building

Dependency management is a bit special, supporting testing against local

development packages.
When building a package, we pull dependencies from

reviewboard/dependencies.py and serialize them to

package-requirements.txt. This file is then consumed by the underlying

setuptools backend, turning into the list of dependencies for a package.
When installing in editable mode, it goes through the list of

dependencies and looks for any that has a matching symlink in

.local-packages/.
If one is found, then that path is used for the dependency, allowing us

to build and test against a local package. We do this automatically for

Djblets when invoked by ./contrib/internal/prepare-dev.py.
The result is then used as the list of dependencies for the temporary

build environment, and for the .npm-workspaces symlinks.
Static media setup uses .npm-workspaces as before, but it's now built

automatically when we pip install -e or build a package.
Note that any symlinks in .npm-workspaces will get replaced with links

to temporary locations when building a package, but this is all the more

reason to only build a package in a clean clone (beanbag-tools does

this automatically).
Along with these changes, MANIFEST.in has been updated with several

files and exclusions needed to generate a suitable source distribution.

We previously had files included that should not have been there and

other files missing. The list is now more comprehensive.
Given the straightjacket that is the Python build environment, some of

these specialized steps are very non-standard. Though this should not

break, it does require further testing. We'll aim to ship this as part

of Review Board 7.1.

Tested setting up a new development tree using prepare-dev.py with a

local Djblets tree (and a public build, but we can't build static media

with that, lacking shipped build tools in the Wheel). Verified that I

could run Review Board and all static media loaded, and that the links

in .local-packages and .npm-workspaces were correct.
Tested manually setting up editable mode without prepare-dev.py.
Tested building wheels and source distributions (together and

independently), and compared to the setup.py-based builds. This was

tested in both a clean Git clone and my development tree. I checked

the files in the builds for expected content.