Buildthings is a setuptools-based Python build backend that makes it

easier to work with projects with more complex build requirements,

particularly those involving NPM or packages that may still be in

development in local source trees.
It's based on the build backends we introduced in Review Board and

Djblets, but more refined and capable of being expanded for additional

use cases down the road.
This initial version supports:


.local-packages/, mapping dependency names to local source trees,

  used in isolated build environments (available per-build type).


Options for including dev- and install-time dependencies within the

  isolated build environments (available per-build type).


An option for excluding certain, more complex dependencies from the

  isolated environments (available per-build type).


Options for enabling NPM installs and for managing workspaces based on

  other Python packages.


Support for extra build steps for editables, wheels, and source

  distributions.


Much of this behaves the way it did in our in-tree build backends, but

with several behavior issues fixed. For example, development

dependencies will now map to local paths if found in .local-packages/.

Further, depending on an editable package using this backend will no

longer error out when it in turn depends on other local trees.
Full documentation for all the options are available in the README.

Converted Djblets and Review Board over to buildthings.
Tested building in new virtualenvs using editable mode, ensuring

that .local-packages was picked up, and using the install deps

options.
Checked that when building Review Board, the Djblets .local-packages

was used.
Verified that the local packages weren't installed as package

dependencies (which would override any editable installs).
Tested various combinations of the options and overrides.
Tested building sdist and wheel distributions, with and without

.local-packages and other deps turned on.
Tested the build steps support.
Tested that when building packages with include-{dev,install}-deps,

those packages were installed in the environment but that the

development deps didn't become part of the final package's dependency

list.