We now support building extensions entirely based on a pyproject.toml.

This will be the standard going forward, and will require the following

configuration:

[build-system]
requires = [
    'reviewboard~=<version>',
    'reviewboard[extension-packaging]',
]
build-backend = 'reviewboard.extensions.packaging.backend'


If there's an existing setup.py, then things will continue to work

as-is. If one doesn't exist, it will be generated during the lifecycle

of the build, calling into our own setup() method. This is the same

process that setuptools itself uses when building packages, since it

still uses its own machinery under the hood.
rbext create will now generate this file automatically, and no longer

generates setup.py.
The documentation has been updated to reference pyproject.toml and

show examples, replacing all old setup.py references, old references

to eggs, and old command line examples, in an effort to help modernize

some of this.

Generated new extensions and built them without issue.
Added a bare-bones pyproject.toml to an existing package, specifying

the new build backend, and verified it built.
Built the docs and went over the changes, making sure they didn't appear

broken.